I've always liked this write up over at leangains by Martin Berkham
An extra 10 lbs added to your bench isn't impressive if you also gained 10 lbs of body weight. That's a 1:1 ratio of weight to strength, which is strongly indicative of fat gain. However, assume you added 10 lbs to your bench, but only 3 lbs of body weight. That's a 1:3.3 ratio of weight to strength, which is quite good. Odds are most of those 3 lbs came in the form of muscle and not fat.
Setting up specific guidelines to strive for in terms of weight to strength ratio is hard and has been a pet project of mine for a long time. While some very general guidelines can be set up for the average guy of average height and build, there are differences between body types.
Tall and long-armed individuals, such as myself, will excel in pulling movements while suffering in pressing movements. For them, modest weight increases usually result in a lot more weight on the bar on movements such as the deadlift. Conversely, they will always see lower gains in pressing movements and usually have to gain a substantial amount of weight to get their bench moving. The reverse conditions apply to short and barrel-chested individuals, which excel in pressing movements but suffer in pulling movements.
Another confounding factor is training experience. It's easier to get your bench press from 200 lbs to 250 lbs without gaining a ton of weight, but harder to take it from 250 lbs to 300 lbs.
For the intermediate lifter, a category I think the majority of my readers would fall into, strive for the following weight to strength gain ratios:
Bench Press and Weighted Chins* - 1:3
Squat - 1:4
Deadlift - 1:5
* Example, weighted chins:
Body weight (180 lbs) + 25 lbs x 6 = 205 x 6 to body weight (185) + 35 lbs = 220 x 6 is a 1:3 ratio. 5 lbs weight gained, 15 lbs strength gained.