Black Jack said:
Seedless raisins are hugely high in calories and also chewy so will give teeth some exercise. Bananas and sweet cherries are also high in calories - again a bit of a Marmite food in that some love them some hate them.
Meant to put in post above adult at 9 months not 8 - oops sorry
You want to give them a healthy, balanced diet; not just one full of calories to "fatten them up". 3 months is nowhere near fully grown, some pigs won't reach their adult weight until as much as a year 'til, like Lez says, 15 months.
Too many bananas can cause serious constipation. Raisins are high in sugar, as are cherries. Piggies do not need large amounts of carbs.