Maybe you and others had doubts. But it was VERY clear from his performance in big - what his range was.
That's not at all true. Big was not a dramatic role, it was a comedy with the occasional cheesy, forced heartfelt moment. It was obvious from Big that Tom Hanks was an amazing comedic actor, but not dramatic actor. Those are two extremely different things, and there are countless great actors who have been terrific at one and terrible at the other. Philadelphia is a very heavy drama about what at the time was a very serious, controversial, tragic issue. Big is a silly, lighthearted film about coming of age and discovering that growing up is about more than just being taller. When Hanks was cast in Philadelphia, people were shocked, and it's not because anyone doubted that he was intelligent or that he had talent. It was simply a very new role for him, which no one had ever seen him do. In fact, I'd say that the closest he had ever gotten to serious drama at that point was when he played the drunk uncle on Family Ties. (It's still one of my favorite Hanks performances.)
This is a lot like the time when Michael Jordan announced he was going into baseball. Everyone seemed skeptical. No one doubts that Michael Jordan is an amazing athlete, but being great at basketball doesn't mean he'll automatically be great at baseball. And he wasn't.
Lets hope Ashton Kutcher is more Tom Hanks and less Michael Jordan in this case.
UPDATE: Actually Sleepless in Seattle came before Philadelphia, and A League of Their Own before that. It was really a stretch from 1992 to 1994 where his career took a sudden curve in a completely different direction. His goofball comedies of the 80s, like Money Pit and Turner & Hooch, gave way to more serious roles, first to sports drama and romantic comedy. Philadelphia is probably still his most serious, somber role. Looking at his IMDB page, I'm actually surprise by what little comedy he's done since.