If I run large plates in a medium carrier say a mayflower mbav will that work or will there be no room?
I know the pockets run a bit big but large is a bit big, xl I swim so this has gotta be it.

Depends on carrier. Carriers based on SAPI specs fit only that size plate. Since 10x12 plates have no standard, the carriers have to be generic cut, one size fits most.

The base of the MBAV is an APC, and for plates you need a S/M for sapi small, medium, and 10x12.  The L/XL APC fits L and XL plates.  I have a L/XL APC and it is a pretty tight fit on a set of level 4 large sapi cut.  My guess would be they wouldn't fit.

Hate to break it to OP, but he's right.  If it's designed to accept plates and soft support panels, there is a little more room.  A SAPI carrier is a pretty safe bet as most of those will size up/down... a generic 10x12 isn't ideal, and a proprietary system or weird swimmer cut shapes are, well... proprietary.

Something else to consider is the method of retention.  If it's not a cheap (and unreliable) velcro folded-seam closure, it will have internal retention as well.  
Is the plate held in place with an adjustable velcro-nylon strap, is it held in place with just a simple velcro seam closure, is there a size-specific elastic strip that hold it in place?