Just look at my sisters Nerys and Nia; Nerys is about 200g (7oz) heavier than more active and less greedy Nia!
A firm belly is OK. A bloated belly is painful, and the piggy won't like it to have it touched. It also goes with other signs, the main being loss of appetite and problems with pooing as food is no longer processed/passed through.
If you feed "gassy" food like cabbages, cauliflower, broccoli and kale as one part of a serving of five different veg/fruit a day, you should not be in any danger, as you won't feed big amounts at once.