It's from Rippetoe in Starting Strength.
On the other hand, if you’re a little fluffy around the belly, you have obviously already created the conditions necessary for growth. You’ll start out stronger than the skinny guy, and strength gains have the potential to come easier for you if you eat correctly because your body hasn’t got the problems with growing that skinny guys do. You still eat a lot, but you don’t drink the milk and you cut your carb intake if you don’t see bodyfat levels drop during these first couple of weeks.
So, if your squat didn’t go up 40-50 pounds between the first and the sixth workout, you’re either not in that demographic or you’re not doing the program. If you’re one of these guys who thinks you gained a lot of strength because your squat went up 30 pounds in three months, you’re not doing the program (which I shall henceforth abbreviate as YNDTP, to save myself some time). If you think the program is hard because your bodyweight at 5’ 8” went from 148 down to 146 and you got stuck on the third workout having gone up 15 pounds, YNDTP. If you’re a fat guy that has decided to go on the Atkins diet at the same time you started the novice progression, are continually sore, and are stuck at 30 pounds of squat increase, YNDTP.
After the first couple of weeks, the increase at 10 pounds per workout becomes unsustainable and 5-pound jumps become the rule. This provides for a long, steady linear increase in strength that has the potential to go on for months. It translates to a 15-pound per week increase in squat strength, half as fast as the first two weeks but still very significant at about 60 pounds per month. This adds up to a 225-245 x 5 x 3 squat workout after 6-7 weeks of training for our novice male, IF HE HAS BEEN EATING CORRECTLY. And eating correctly is part of the program. If he started the program at a bodyweight of 165, he should probably weigh 185 by then, more if he’s taller. If you’re squatting 30 pounds more than you started at six weeks into the program, YNDTP. If you started at 5’ 9” and 155, and six weeks later you weigh 160, YNDTP. If you started at 5’ 9” and 235, and six weeks later you’re only squatting 50 pounds more than you started with at a bodyweight of 235, YNDTP.