excuse the dodgy typing , I'm holding my lardy hog GP5 - Ijust told her she's looking tubby and now I'm being bitten 
Grass does make little piggies tubby - like little native ponies, they get fat on lush grass when they should be eating sparser grazing and walking more to find it. but we domesticate these animals so they evolve to accept the spoiling.
Grass and hay is their natural diet so should be freely available unless you have young pigs or they're going out on spring grass after winter and you don't want them to gorge and bloat themselves.
What you could cut down on is fruit (high in sugar) and root veg - carrot,parsnip all the sweeter things that are good to fatten up thinner pigs over winter.
If you leave the run longer in one place they'll naturally reduce their grass eating? I move ours every 3 days but they're a pain in the neck to remove the pegs, move them and secure the pegs again.
If mine have a grazing day they have supper of hay,pellets and limited veg.Though they claim that mowing the lawn makes them hungry because it's hard work.
Weigh them weekly.
Keep a close eye on their droppings
we do a multi purpose mite and wormer drop (Xeno 450) because I have no idea what random animals wander about my garden.
And yes, they'll have chins it goes with the territory and 5 is Piggie Middle Age.
For their own health and happiness , (their joints and to reduce the diabetes risk and strain on their hearts) they shouldn't carry too much weight but they are little vegetarian eating machines ; my DD says her boars 3 loves in his life is : DD, Food, His Wives.