Great to hear, I am sure they will love having a whole day to run and play!
I think maybe a switch in pellets could be tried - if you can afford it, see if they will take to Oxbow Cavy Cuisine. It's based on hay, unlike Supa Excel, and I found my pigs ate far less of the Oxbow than any other pellet; presumably it was a bit more filling so they didn't need to eat much of it.
I do wonder if perhaps Ches was undernourished/underweight when you first got him, due to his past. Perhaps he didn't reach his full size when he was younger due to a poor diet and he's catching up now his diet is good and healthy...?
Maybe he is naturally a small pig, but even dainty pigs tend to stop gaining weight at 10-12 months or so. Veggies don't normally cause weight gain, although sweetcorn/babycorn can be quite fattening. Splitting the 125g of veggies up into two meals, and not feeding anything in between, might be a good next step. The best trick I found for encouraging a slightly overweight pig to tone up, is to spread the veggies entirely around the floor during floor time. They have to move about in order find their food...it worked a treat with Dexter when he started to tip the scales at nearly 1600g.
Maisie sounds just great, she sounds a lot like my Meghan who is also 1225g!
It is slightly puzzling as there are no clear reasons for Chesney's continued weight gain: his dry food is limited already (which is good), he gets some good hay and a nice varied fresh veggie diet, and he's getting plenty of opportunities for exercise.
As far as I can see, you are in no way at fault here and you have no reason whatsoever to feel guilty. You have dealt with the most common reasons of excess weight gain, which is more than many would even think to ask about. Chesney and Maisie live a privileged life, they are lucky to have found such a devoted, loving home and human! Please don't feel bad. *hugs*