Spider-man shouldn't be bulky, but that's not the same as being skinny. Andy Garfield looks skinny, not lean. Tobey Maguire looked lean. When I look at the most classic illustrations of Spider-Man, the impression I get is that he is someone who is very athletic and agile, not that he is a shrimpy guy who looks awkward in tights. Peter is not physically large but that does not mean that his musculature is downplayed.
Well, first of all, using comic illustrations doesn't really prove your point because everybody is incredibly muscular in comics. Despite the fact that Spider-Man is drawn like an amateur bodybuilder, in comics that translates as 'skinny' when compared to most superheroes who give Ronnie Coleman a run for his money. Just look at Jim Lee's Superman;
Look at those forearms. Those alone are probably bigger than Henry Cavill's bicep.
It was not really until Ultimate Spider-Man came along that there was any significant push by Marvel's artists to depict Spider-Man as being skinny with a huge noggin.
Or, say, Steve Ditko's depiction of him in Amazing Fantasy 15;
Ultimate Spidey looks the way he does because Marvel wanted to play up the fact that in Ultimate Spider-Man, Peter Parker is just a kid.
Which is exactly what Spider-Man was back in 1962. People forget that Spider-Man was created as a teenage superhero.
Making him skinnier with more child-like proportions helped differentiate the Ultimate version from his more adult looking 616 counterpart, who is in his latter 20s and has been Spider-Man for quite some time.
The thing is, Garfield is playing a Spider-Man that is still in high school, due to ASM being an origin story. So why should he be built like a Spider-Man in his latter 20's?
I know that Steve Ditko eventually bulked up his Spider-Man, because unlike today, Stan Lee actually wrote his characters as developing human beings who aged and experienced changes in their life. He was 15/16 in AF 15 and 20 by 1966. Spidey simply filled out to look more like a gymnast. Now he's stuck in a perpetual state of 'not quite 30' and they have to keep him young by making deals with the devil to wipe out his marriage, but I digress...
Not that the Ultimate version really works in Garfield's favor that much, since he is a tall, lanky 30 year old rather than a short and scrappy looking teenager, so the only thing he really has going for him is the big egg-shaped head-- assuming that the end goal is to look like the Ultimate version.
Garfield may be 30 years old in real life, but he looks much younger, which is why his breakout role was as a college student in The Social Network. He is perfectly convincing as a teenager from the images of ASM that I have seen, and he looks far more like the Ditko version of Spider-Man than Maguire ever did.
As for being tall, Garfield may have been listed on some sites as 6'0 but on CelebHeights he is barely 5'11, which is tall-ish but not far off of Spidey's given height of 5'10.
He's less taller than Spidey than Cavill is shorter than Superman, in any case.