(26f) bf wants a threesome with a smaller girl

my bf wants to have a threesome and I think it’s pretty hot, but the girl he wants to invite is so petite we’re like complete opposites I’m 5’4 while she’s 5’0 I’m scared my bf will end up liking her more me, but the sexual thought of it is so appealing to me I’m not sure if I should go through with it I don’t want to feel worthless or something during it, all my bf’s exes were small woman so I’m hesitant about introducing her to our bedroom.