It is the design and construction of the head. If the head is not large enough it physically cannot cover the larger negative. Example you cannot print anything larger than 6x6 in a B22 enlarger, the head is not big enough to cover a larger negatives. With the larger head of a D2, you can print anything up to 4x5 inches.

I found the info on your enlarger. I thought the B-series maxed out at 6x6, but according to the KBH site, your B5 will handle 2-1/4x3-1/4 or 6x9.
You just need the appropriate negative carrier, lens mount and lens. That could be difficult based on how old your enlarger is, but you might get lucky.
Start looking.