Still find myself watching Friends from time to time. I was a big fan of the show during it's run and considered it must see TV (no pun intended). Matthew's weight gain is noticeable over the series in the time frame that Spot's Meow mentioned but I don't really notice that much of a difference in his actinng. Trying to remember what happens in Sesason Seven...for some reason I want think that was Chandler and Monica's wedding so if it is then it's only reasonable that Matt's acting was more hyper since there was a whole arc dealing with Chandler not being able to accept being married.
Well I haven't rewatched all of season 7 yet so I don't remember how Perry does in the rest of it. I'm mostly basing my opinion on the last episodes of season 6 and the first episode of season 7. Perry's acting just isn't as good in the season 7 premiere, he looks nervous and the comedic timing is what's really off. He was going through a lot at that time, just getting out of rehab and dealing with other issues so I don't blame him really, and someone who hasn't seen the whole series probably wouldn't pay much attention to it, but after watching episodes back to back on DVD it did become noticeable to me.
And yes, they were getting married...in the season 6 finale Chandler proposed and then the season 7 premiere was supposed to be immediately after that, with all the friends wanting to go out to dinner to celebrate, so his change in attitude from the calm to the hyper doesn't make a lot of sense in that context.