Some additional observations:
Gigabyte's taiwanese FTP site mirror at archive.org has a list of the BIOS versions, but none of the download links work.
However those files have been mirrored at mobokive and it seems all the LX3 ones are 128k!
Seems like an oversight on Gigabyte's part.
So wimsbios have probably dumped that 256k file themselves.
The sibling model 686LX4 has 256k and it's bundled with either AwdFlash 6.6 or "Flash Utility Ver.836", whatever that is.
So I don't think this 1Mb => 2Mb conversion is managed by their flashing tools (I could be wrong tho!)
The initial BIOS version (6lx4101.zip) for LX4 is actually 128k and they have a one-liner about it in their v1.03 update.
It seems the 256k version is rare (the wimsbios is the only one I spotted and it doesn't have direct dl), so could you post your dump here?
Maybe it can be patched if there's some particular problem you want to get solved.
(but it needs to be done by somebody more knowledgeable than me 🤣)
EDIT: OK it seems if you combine 128kB worth of zeroes followed by the 128k BIOS, the combined file will run in an emulator (with a mobo model that has 256k BIOS) 😁
Maybe some flash tools can do something like that on the fly (actually could be a bit risky to have that feature though).
The 256k versions on Gigabyte site ofc have actual content in that extra space, and not just zeroes.