In the Bentley Layton translation:
Jesus tricked everyone, for he did not appear as he was, but appeared in such a way that he could be seen. And he appeared to all of them--he [appeared] to [the] great as someone great, he appeared [to] the small as someone small, he [appeared] [to the] angels as an angel and to the human beings as a human being. For this reason he hid his discourse from everyone. Some saw him and thought they were seeing their own selves. But when he appeared to his disciples in glory upon the mountain he was not small, (for) he became great; or, rather, he made the disciples great so that they might be able to see that he was great. (57,28-58,8)
In this passage, Jesus rarely appears (if ever) in his true essence, which is inaccessible to most beings. He appears as like the one who sees him. He acts as an ultimate Rorschach--one sees themselves in him. He is almost a mirror, "they were seeing their own selves."
This is because, according to the Gospel of Philip, only like can see like:
People cannot see anything int he real realm unless they become it. In the realm of truth, it is not as human beings in the world who see sun without being sun and see the sky and so forth without being them. Rather, if you have seen anythings there, you become those things: if you have seen the spirit, you have become spirit; if you have seen the anointed (Christ), you have become the anointed (Christ); if you have seen the [father, you] will become the father. Thus [here] (in the world), you see everything and do not [see] your own self. But there, you see yourself; for you shall [become] what you see. (61,20-34)
Combining the two sayings: you are what you see, and see what you are. What you see in the realm of truth will be yourself. As such, the disciples could not see him in glory unless they too have been transformed to see it. They see it, and discover they too are great. Thus, according to the Gospel of Philip, Jesus does not change on the mountain; the disciples do. They finally see what was already always there.