Believe it or not, up to a certain point (and it's pretty far up there), there is no positive correlation between muscular strength and muscular size.
If you want to put on size and are already performing activities conducive to strength gain, eat more. It's very likely that simple. As a skinny person/hard gainer, I wouldn't even really worry about counting calories or anything, just
eat clean and as much as you can. It might still be helpful to check out some calorie information, though, just to get an idea of how much is enough. Chances are you're eating at or near your maintenance and just need to eat more.
Then again, since you say it's only been a month, it's also possible that most of your strength gains have come as the result of neuromuscular adaptation, for which there is an adjustment period before your body will actually begin to spend precious energy on increasing muscle mass. (I'm sure you can see why, biologically, this makes a great deal of survival sense).
So, I'll amend my previous suggestion to include, "Be patient." Eat more, and relax.
Edit: If you are eating enough (and you're absolutely sure of it), trying getting more rest (both sleep and between weight training sessions) and/or spending less time in the gym (working at a higher intensity). Maybe check out
Westside for skinny bastards.
