Will biking like this make my legs bulky?

So, I'll be biking to work in the summer. The ride one way is about 3 miles, so that's not too bad but theres like MASSIVE hills. Really steep, reallly long. Are my legs going to get bulky? Is there anything I can do to prevent it? I usually keep my gears (the 1-3 ones) on 2, and the 1-7 ones on 7.