Yeah - I think there are a lot of little "factors" at play here. I know someone who spent time on broadway trying to make it as an actor get into movies. People don't realize that these people are often selling how they look - so they spend a lot of time on their body to begin with. A lot of them aren't ever really that out of shape, they may carry extra fat, but they're working out. I'm not talking everyone - there are plenty of Jack Blacks - but it's the industry.
And of course, if you had a role you were going to make $1M to play (or much more) - you'd get some "assistance". It wouldn't be professional not to use the drugs at your disposal on some level.
Plus you'd have good coaching and help.
But I think the BIGGEST factor is that this is their JOB. If you or I go on an extreme hardcore diet and exercise regimen - fine, we will see fast results and it will be hard - but we will be fitting it in between our daily responsibilities of family, day jobs, etc. We only have so much energy to give to life. For the actor, it IS their day job.
Just think if you woke up tomorrow morning and someone said, "OK - you're done with work. I'm going to pay you $1,000,000. All I need you to do is drop 50 pounds in the next 2 months and learn these lines." What would you do? Of course, yeah - getting in that position takes some work, pitfalls and acting skill - but you'd probably make an amazing transformation too.