Then you defeat the entire point of Unicron if you have to resort to him sending miniaturized effigies of himself. The big deal (and indeed problem) with Unicron is that he should be a planet sized robot. A scale so freaking massive that interaction on the individual scale should be impossible- that scene in TFTM when Unicron eats Galvatron? Galvy would have had to roughly be the size of GREAT BRITAIN, a literal entire COUNTRY, to actually be that big during that sequence. That's why Unicron typically never gets involved himself until the end, you can't really top a planet sized robot dragging city-sized fingers through a planet's surface. I mean, sure, Gurren Lagann did but that final battle STARTED with planet sized robots and got to the point of hurling literal galaxies like basketballs by the end. By simply having Unicron generate miniature versions of himself instead of using proxies IE his various Heralds, he becomes nothing more than some generic big monster threat - at that point all that's happened is this franchise becoming Pacific Rim except Prime would simply get a bigger weapon than what he's got and he'd now them all down anyway.