For the past couple weeks I've unintentionally kept my workout exactly the same. Same reps, same weight. Not a lack of motivation, it's just when I'm done I can't do any more. But the crazy thing is, my muscles seem to be getting bigger! So, I realize that I won't get any stronger without increasing my workout, but should I be getting bigger?
Take a look at this question for an indirect answer to your question. Basically, muscle size and strength do not always go hand in hand. Strength is the ability to do work--and you will never get stronger if you never increase weights. However, you can get bigger when you increase the volume of work with lower weights.
Also note, that beginners tend to see bigger/quicker gains than people who have been exercising a lot longer. The main reason is that the difference between your next potential and your current ability becomes a lot smaller.