That's to be expected, of course. If you tweet often, you're leaving lots of keywords out there to be found. Also, the more you tweet, the more often the followers you do already have will see you. Therefore it's more likely you'll get a retweet or mention. If the people who do this have a lot of followers, then many of them will see your profile as a result, and some of them will decide to follow you.
Needless to say, you shouldn't just get obsessed with the quantity. You've got to tweet things that are interesting and useful. As well as getting more followers, you've got to maintain a good reputation as someone who is worth following because you're a consistently good source of information.