Gus
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.