Uchi Mata harder on larger guys?

I find doing uchi mata, either in practice or competition, really fine and easy against people my weight (roughly 60 kilos). But when I try it, even in uchikomi, on a guy that is say 100+ kilos, I can't even remotely lift him off the ground without straining a bunch. Does this mean my technique is bad and the only reason I'm landing it against people my weight is because of strength? Or is that just a common factor with uchi mata?