But there's a way to achieve the effect of that size:

Stand them with Legends/Legion-class figures instead of the normal-size versions!

This is a scaling solution that's even been employed officially. The Gigantion characters from Cybertron are Brobdingnagian by Transformers standards in the show, but their toys are of course still just standard size classes. So Takara included a Legends-class Optimus Prime with the Japanese "Megalo Convoy" release of Metroplex.
And during Combiner Wars itself, there was a Legends-class version of Shockwave to replicate Bruticus taking advantage of being "step on everyone" size to wield Shocky's gun mode (which, unlike Megatron's, does not mass-shift):

Another thing you could do is stand the individual members with buildings that are 1:32 scale (or thereabouts), and then the combined modes with buildings that are, say, 1:144 scale (or thereabouts).