The mask represents something he is appearing to be but is not. Over time, however, the mask becomes who he is since he has taken on this persona for so long...it eventually actually DOES become part of who he is in reality.
In Orwell's opinion, becoming a colonial master means that you have to wear a mask in terms of the role that your perform and also how you are perceived. In this essay, he realises that the crowds of people that have turned up view him as a kind of magician who is going to put a show on for them. However, the problem with this mask is that you have it on for so long that when you try to take it off you realise that your face has actually changed or grown to fit the mask itself, and therefore you can't actually remove that mask any more.
Since your question asks for a personal opinion, I'll give you my opinion of what this statement means. You have to interpret it in its context:
Here was I, the white man with his gun, standing in front of the unarmed native crowd--seemingly the leading actor of the piece; but in reality I was only an absurd puppet pushed to and fro by the will of those yellow faces behind. I perceived in this moment that when the white man turns tyrant it is his own freedom that he destroys. He becomes a sort of hollow, posing dummy, the conventionalized
figure of a sahib. For it is the condition of his rule that he shall spend his life in trying to impress the "natives," and so in every crisis he has got to do what the "natives" expect of him. He wears a mask, and his face grows to fit it.
The narrator represents Imperial Britain, and as such the people expect him to behave as their caretaker. At the beginning of the story, the narrator says that many people hate him, "the only time in my life that I have been important enough for this to happen to me." They hate him for the tyranny that he represents, but they turn to him for help in a crisis. That is what Orwell means when he says that "when the white man turns tyrant it is his own freedom that he destroys."
Orwell uses this metaphor to describe transformation. He means that he has forced himself to act a certain way, to be a certain person that is not who he thought he was. Eventually if we act a certain way we run the risk of actually becoming that person and that is what he is saying. If you wear a mask, if you act like someone you're not, then you will grow to fit, you will become that person whether you like it or not it's almost out of your control if you aren't careful.
Note: a metaphor is a figure of speech in which a comparison is made where one thing is used to designate another making an implicit comparison.
As you ask for a personal opinion, I'll give you mine: I think it means that that there is some standard or lie that "he" is hiding behind. Seeing as though I cannot find the exact place or context of these words, I say that he is either hiding behind it, or making it for himself, only to have to grow into it; grow into something he doesn't belong in, or something he isn't supposed to be in.
Lie: Him growing seems to me like expanding...like expanding a lie. He now must expand everything to fit the lie, or even the pseudonym, in order to make everything work for him.
Hiding: I don't think hiding is the proper term, but basically the idea that he is not in his right place, but also not leaving his current position.
"He wears a mask and his face grows to fit it."
This means to me that "he" must alter something in his life to make it where it "ought" to be.