Compared to Xehanort Sora may have a weak heart. But if Sora's heart was weak, then how was he able to create Roxas. Remeber that it takes a strong heart to have a strong nobody. Look here:
The rest of the organization members turned into rQ. Besides Xehanort, did the other 5 disciples that turned into Heartless disappeared?
A. Not really disappear but more like they're somewhere else as normal Heartless. Xehanort was about the only one who was able to keep a human form.
See the organization had strong hearts as well but they still had weak heartless because of the information that RikuHeroOfDark has provided.
This must be true for the rest of the organization, they are out there but just has weak heartless.
So you can't really say that Sora's heart is weak because that means that the whole organization members had weak hearts when they were human, exclude Xehanort. Xehanort was a special case anyway, he had unusual powers.