I'm actually going to be the voice of dissent here - with the caveat that I've bred a lot of big horses, and not minis.
However, these mares look large - but not necessarily close to foaling. In my experience, udder development is meaningless.
I've had experienced broodmares that bag up like dairy cows far before they're close to ready (not due to placentitis, just because their bodies know what's coming) and I've had maiden mares who didn't develop an udder until after the foal was out and their body realized what was happening.
What I *would* watch for is a deepening V in their bellies and a loosening of their butts, for lack of better phrasing.
When a foal moves into position to be born, they're basically sitting on the bottom of the mare's belly, prepping to swan dive on out. This typically creates an abnormal shape on the mare's belly where they're a little less round and a little more angular than usual and in some mares can be quite a dramatic V shape.
The other thing their body will do, udders be damned, is loosen everything around the birth canal. I know you can't really get your hands super close to these girls yet, but in most mares when they're super close to foaling you can almost bend their tails up over their backs. The elasticity has to come in order for a foal to be able to make their exit - a lot of times that elasticity will show up with a complete muscle drop around the tail head.
Here's a photo of one of my mares a couple of years ago - two days prior to foaling one of the biggest fillies I've ever had. This is a mare who bags up early and could probably milk three foals with her udder. The edema on her belly had been there for a couple of weeks prior to her actually letting go. You can see though how her belly is dropped down with the foal sitting more down; you can also see where the muscles around her tail head have dropped off and her tail looks very prominent.
Someone online (not sure who to give credit to) made this great graphic of a foal overlaid on a photo of a mare, if you're a visual person when we talk about the foal moving into position:
Instead of having a rounded, normally formed "hip" - they can get very angular. It's hard to see the extreme in my picture of the white mare, so I'm borrowing this photo with the source linked below. You can see how, due to the relaxing of the muscles around the birth canal - the mare has gone from having a very normal, rounded hip, to having a very flat, angular, almost emaciated looking hip. This can be a clear indicator that your mare is getting ready to foal.
(Source:
Normal signs of behaviour before and including foaling in mares - Breeding - Horsetalk.co.nz)
Best of luck to you! If they do end up being pregnant, we demand photos!