Theophylact of Ohrid

Now He said not this as ignorant of their answer, but wishing to induce them to tell Him how much bread they had, that so a great miracle might be manifested through their confession, when the quantity of bread was made known.

Catena Aurea by Aquinas ↗

Ambrose of Milan

For we read that first five thousand are fed with five loaves, then four thousand with seven loaves. So let us seek the mystery which the miracle represents. Those five thousand, like the body's five senses, seem to have received from Christ food similar to physical food. But the four thousand are still in the body and in the world that is known to be of four elements.… Seven baskets of fragments remained from the four thousand. This bread of sabbaths is no ordinary bread. It is sanctified bread. It is a bread of rest. …

Commentary on Luke ↗

Bede

But he said to them, "You give them something to eat." He challenges the apostles to the breaking of bread, so that, as they declare themselves not to have anything, the greatness of the miracle might become more known, at the same time suggesting that through them our hungry hearts are to be fed daily. For what does Peter do when he speaks through the Epistles, if not to fill our poorly hungry hearts with the nourishment of the word? …

On the Gospel of Luke ↗

Bonaventure

But thirdly, as regards the scantiness of food, it is added: But he said to them: You give them to eat: you, as their prelates, according to that word of First Peter five: "Elders, feed the flock that is among you." But he says this so that from their response the scantiness of food might be known. — On account of which it is added: But they said: We have no more than five loaves and two fish. …

Commentary on Luke, Chapter 9 ↗