Because Unicron's size is something only a cartoon can deal with. Grimlock is big enough to be seen biting Unicron's skidplate on a reasonably sized zoom out, even tho Unicron is so big that it looks like him standing on Cybertron is like one of us standing on a beach ball. When you take into consideration that Cybertron has got to be bigger than earth, you'll can imagine how off Unicron's relationship in size to other TFs is. This doesn't matter in a cartoon. But in a CGI live action, the fact that they stick to a size ratio would ruin it. It'd make Grimlock roughly the size of the United States of America. As you said, he's too big to be handled correctly by a live action movie.