The bottom line is that your strength gains will not be their best unless you are in a caloric surplus. I trained my first 5 weeks of StrongLifts at the same weight and in a caloric deficit, and I saw very little muscle mass gain, if any. My strength did increase, and what muscle I did have became more defined; however, my strength gains started to taper off and each week and the lifts became more of a challenge. In fact, the same calories became insufficient to maintain what I was beginning to build, so I started to actually lose weight at that calorie number. Ultimately, I changed my diet to keep up the pace of gains. I had to. I now eat in a slight caloric surplus, and I surprise myself every week with what I'm capable of. Honestly. And with the additional muscle mass, I find it
very difficult to keep the weight gains every week. It gets harder and harder.
The hardest pill to swallow is knowing that as you get "big," not fat, it gets substantially easier to lose any weight you put on because your muscle size increases. Simply put, bigger muscles mean quicker natural weight loss. It's that straightforward. You burn more calories with more muscle mass. Keep this in mind as you get stronger, because you have to adjust your diet accordingly. This is why resistance training is important for women as well, even though some are hesitant because of potential muscle size gains. But potential is the operative word in that instance.
In the end, know that what you
should do depends on your goals. Do you want to be really skinny but really fit? Do you want to add a little muscle mass or significant muscle mass? We can give you more definitive answers knowing this. But, for now, know that you can bulk up slowly, but also know that how slowly you gain body mass can be proportional to how slowly you gain strength. I maintained the same weight for 5 weeks of training, and it impaired my strength gains. I wanted every hour in the gym to be fully taken advantage of, so I had to adjust my diet. I now try to put on half a pound to a pound a week. Presently, I surprise myself each week with what I'm capable of, and that's due to a caloric surplus. And I'm still skinny to average in weight. I am in no way fat, but I can't deny that my bodyfat has gone up slightly, but that is accredited to very high carbohydrate foods -- it's a challenge to eat clean and easily meet a caloric surplus, for me.
The last thing worth note is the Cunningham equation. It helped me find that point where I eat in a caloric surplus without going overboard. Or, in other words, eating enough for strength gains without putting on much fat.
http://stronglifts.com/forum/the-cunningham-equation-an-accurate-way-to-predict-calories-t13712.html