Not sure if you mean base size or if you were just looking at how some warjacks seem to tower over others. human sized models are on 30mm bases, light warjacks, trolls, orgres and other large humanoids are on 40mm and heavy warjacks/very large creatures are on 50mm
some of the unique jacks tower over the standard heavy jacks i think its mostly to make them look cool/grandiose in their poses.
PP advertises is as 28mm or '30mm' scale. I just put Deneghra next to a GraveGuard and they're pretty much spot on the same size. 40K minis long ago broke the bounds of anything that resembled 28mm scale. WarJacks do scale a little bit oddly. The humans are more or less equivalent in scale to WHFB, but a lot of heavy jacks that are supposedly 6 ton iron behemoths look like they could get totally worked over by your average Ogre Bull.
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