In that specific instance, there is honestly not that much of a size difference. Simply put those Termies on a 32mm base, then put the 40mm base underneath that. Cover it in your basing material of choice and you're done!
Like so-
Now, the metal Termie on the far left was embiggened a tad by cutting him at the waist, but you can see with the riser on his base he's as big as a modern Termie.