Is it insulting to give an expensive gift to a girl if her boyfriend only give her a small gift?

If she's your good friend/best friend, then no. It's the boyfriend's own fault for not choosing to spend more on his girlfriend. If they've not been dating long and he chose to start out only giving her a small gift then, again, no because if you're close enough to her that you felt the need to give her this more expensive gift, then it shouldn't matter. If you are not very close to her and only have a desire to be closer to her or possibly replace her boyfriend, then yes, it is inappropriate because that's not your place to show up the boyfriend.

This is about your definition of insulting. People have different intentions when it comes to giving a gift, for some it is about the cost and for others it is about the sentiment.

Buy here a bottle of vodka and some orange juice. Get her to drink A LOT of it. Then tell her you got a gift for her in your bedroom she just needs to come up so she can get it. Then give her, her "real" present real good and slow. Tell her this is what a real man gives his girlfriend for christmas.

Giving an expensive give to a girl you are not dating is inappropriate regardless of what her boyfriend bought her.

-No. That's the Girl's "Issue"- not Yours. :)