Heavy people need a firm mattress that will support their weight, whilst providing durability. Sagging needs to be avoided, so bigger bodies should look for mattresses with reinforcement to help prevent this. A firmer mattress also helps to keep the spine aligned and prevent aches and pains.
A firmer mattress is always a good choice for back and stomach sleepers, who need support and structure for optimal alignment. Some side sleepers may benefit from a slightly softer top layer, but in general a heavier body is always going to need the support of a firmer mattress.