Training quads is a bitch.....a lot of times if you are really pushing as hard as you should be, you will get a severe case of DOMS,sometimes for days after training legs as opposed to any other bodypart.
I know people will say my legs sucked which they did, but I ALWAYS trained the living fuck out of them.
I also agree with Jeff.....when it comes to squatting,your lower back is a major weak link.....I was a pretty good squatter for a light small boned guy and they personally never worked well for me regardless of weight being heavy,high reps,low reps,all different kinds of intensity techniques thrown in.....the result in my case is that now I have three bulging discs and arthritis in my lower back and also in my neck.
Sorry to ramble.....my coffee just kicked in. 
It seems to me as if no matter how hard some folks train their legs, they just never get huge or as big as they hope. My quads seemed large in proportion to the rest of my physique when I had a horse and went riding almost daily. They also grew when I commuted to and from work on a bicycle every day, but by that time the rest of my body was more muscular as well, so my quad size was less noticeable because they were more proportionate. Calves are another story. I am convinced the only way to have big calves is either because of genetics or implants. LOL!
My degenerative disc disease is mostly due to my advanced age and definitely not because I did loads of really heavy squats. Same for my arthritic knees with almost no meniscus padding them.
