I will say the WGF Dark Age stuff is much more slender than Bolt Action Stuff. I've built the US
BA infantry and both Rising Sun and Hammer of the gods stuff (plus some Roman stuff). WGF doesn't always scale across their own ranges (WGF Germans are notably smaller than WGF Hammer of the Gods). See below for a WGF cross range size comparison:
As you can see, the WGF German on the left is at least a base shorter than the WGF Viking on the right. The heads and arms are definitely bigger as well, and the bearskin cape was part of me having to hide this size difference. The Bolt Action Americans are similarly thicker and beefier than the WGF stuff. I also found the heads to be a bit big. Arms might work on WGF bodies, but I doubt heads would unless the Japanese are notably smaller than the 'chibi' American heads.
Also see:
http://www.dakkadakka.com/dakkaforum/posts/list/60/430371.page
Major Thom has a scale comparison shot. WGF is again thinner than
BA