Theophylact of Ohrid

Although Peter left little for the sake of Christ, he still calls this little "everything." It is evident that even a little has the bonds of attachment, and therefore even one who leaves little is worthy of being blessed. Peter alone asked Christ, but the Lord gives an answer common to all.

Commentary on Mark ↗

Bede

Peter began to say to Him, "Behold, we have left everything and have followed You." Great confidence—Peter was a fisherman, he was not rich, he sought food by hand and skill, and yet he speaks confidently, "We have left everything." And because it is not enough only to leave, he adds what is perfect: "And have followed You." We have done what You commanded, what then will You give us as a reward?

On the Gospel of Mark ↗

Glossa Ordinaria

Because the youth, on hearing the advice of our Saviour concerning the casting away of his goods, had gone away sorrowful, the disciples of Christ, who had already fulfilled the foregoing precept, began to question Him concerning their reward, thinking that they had done a great thing, since the young man, who had fulfilled the commandments of the law, had not been able to hear it without sadness. …

Catena Aurea by Aquinas ↗