Why is Garuda considered stronger than many Devas?

 Garuda isn't only considered stronger than Devas, He actually is stronger than Devas. His feats speaks for himself.

He alone fought and defeated all the Devas including Rudras and Vasus:

Thus, Garuda brought complete turmoil to the world of the gods. He ripped the gods apart with his wings and beak. Then, the god with the thousand eyes commanded Vayu, “O Marut! It is your duty to drive the dust away.” Then the mighty Vayu drove the dust away. ‘When the darkness had disappeared, the gods attacked the bird. He roared in the sky, like a giant cloud, terrifying all beings. Attacked by the army of the gods, the immensely valorous king of the birds, the destroyer of enemies, rose into the sky, above the heads of the gods. Led by Indra and armoured, they attacked him with many weapons like lances, iron clubs, spears, maces, many sharp swords and chakras as radiant as the sun. Attacked from every side, the king of birds didn’t even tremble. Vinata’s powerful son fought a tremendous battle, showing no signs of tiring out. Like the roar of thunder in the sky, Vinata’s powerful son attacked the gods from all sides with his wings and breast and scattered them in all directions. Oppressed and mangled by Garuda’s beak and talons, a lot of blood flowed from the bodies of the gods and they fled. Overcome by the lord of birds, the Sadhyas with the Gandharvas fled eastwards, the Vasus with the Rudras towards the south, the Adityas towards the west, and the twin Aswins towards the north. Gifted with great energy, they retreated fighting, looking back every moment on their enemy.

Even Indra’s Vajra is nothing infront of him, When Indra Hurled Vajra at Garuda, He easily tanked it and was laughing at Indra.

Garuda, was coursing through the air after wresting the Amrita, Indra hurled at him his thunderbolt. Then Garuda, the lord of birds, struck with thunderbolt, spake laughingly unto Indra engaged in the encounter, in sweet words, saying,'I shall respect the Rishi (Dadhichi) of whose bone the Vajra has been made. I shall also respect the Vajra, and you also of a thousand sacrifices. I cast this feather of mine whose end you shalt not attain. Struck with your thunder I have not felt the slightest pain.'And having said this, the king of birds cast a feather of his.

The only reason, He allowed the thunderbolt to touch his body and he didn't thrashed Indra cause of Respect for the weapon and the deity.

I think it's enough to say, That Lord Garuda is in a league of his own compared to the Devas. Remember Garuda is best among birds as said by Lord Krishna in Bhagavad Gita.

Did Garuda meet Krishna?

 Yes. Garuda was a constant companion of Lord Krishna throughout his avatar.

1. At the end of Govardhana Leela, Indra saw Garuda giving shade to Krishna with his wings -

Disappearing from view, that foremost of birds (Garuda), feeding on serpents, was protecting him, seated at ease, against the rays of the sun with his wings. Leaving behind his elephant, Indra, the slayer of Bala, approached Krishna who was playing in the forest being engaged in human actions.

• Chapter 19, Vishnu Parva, Harivamsa.

2. Garuda placed Vishnu’s crown on Krishna's head -

Beholding Vishnu in a human form on that best of mountains, without any diadem on his head and with out any visible dress and apprised of his intention that foremost of birds threw from the welkin that crown on Vishnu’s head in such a way as if it had been attached to his head from before.

• Chapter 41, Vishnu parva, Harivamsa.

3. Garuda accompanied Krishna to Vidarbha -

Thereupon in order to terrify the kings and display his own prowess he thought of the highly powerful son of Vinata who had attained Siddhi beforehand. As soon as he was thought of, Vinata’s son, assuming a form that could be seen easily, approached Keshava.

• Chapter 48, Vishnu parva, Harivamsa.

4. Krishna and Satyabhama went to Fight Narakasura riding on Garuda -

Then taking up his conch, discus, club and sword, he, along with Satyabhama, sat on Garuda’s back and immediately started with Sakra. Before the very eyes of the leading Yadus, Keshava, assisted by the powerful Garuda, crossed the seven regions of the wind-gods and rose high up.

• Chapter 64, Vishnu parva, Harivamsa.

4. Krishna fought Indra for the Parijata tree while riding on Garuda -

He rode on Vinata’s son and caused terror in Amaravati. The brave one brought parijata from the great Indra’s residence. Knowing his valour, Shakra tolerated this. We have not heard of any king who has not been vanquished by Krishna.

• 987 (10), Dronabhisheka Parva, Mahabharata.

5. Krishna went to fight Banasura riding on Garuda -

After Krishna had sat on Garuda’s back Halayudha and Pradyumna, the grinder of enemies, sat behind him. At that time the Siddhas, Caranas and the great Rishis said to Keshava in the sky:—"O you of large arms, do you vanquish Vana with all his followers. No one will be able to stand before you in the great battle.

• Chapter 122, Vishnu parva, Harivamsa.

6. On Krishna's command, Garuda Sits on Krishna's flagstaff -

Krishna thought of Garuda and he immediately arrived, the instant he had been thought of, like the tall pillar of a temple. Garuda, eater of serpents, then sat on that supreme chariot on the flagstaff, together with many other blazing beings that had their mouths open and roared loudly.

• 247 (22), Jarasandha vadha parva, Mahabharata.

7. Krishna went to fight Paundraka Vasudeva by riding on Garuda -

So saying, he dismissed the, ambassador to report these words to his sovereign; and summoning Garuda, mounted him, and set off for the city of Paundraka.

• Chapter XXXIV, Book V, Vishnu Purana.