I think personally, due to the Internet, EVERYTHING is getting bigger.
However, I'd say over the past few years the broadcast radio/MTV has certainly been pushing for more Hip-Hop and Pop. Rock, even of the most commercial variety, is fading out of that spectrum.
What I think is happening though is you're looking at music as three entirely different audiences.
A) Casual people who look to MTV to sustanance. This group will pretty much accept anything, and everything, shoveled into their mouth. They don't care what it is, but just get into it because everyone likes music.
B) Others who look to MTV but chose only to do it because they really don't have an alternative medium advertised. They will not listen to college radio or go out of their way to find their own niche, so they accept what's given to them and discern in that.
C) People who demand something specific in music. Will only listen to what they like, and will not allow MTV to sway them. These are the people who get into what was known as indy or alternative pre-Nirvana. This is where Punk/Underground culture came from.
In the 80s, pop was big. Pop was a genre that seemingly(even by namesake) was developed solely to sustain an audience quickly, and effectively. Of all musical traits, accessibility and mass producability are the end goals. This pleased audience A, and only slightly sustained audience B, and completely alienating audience C.
In the early 90s there was a shift to Alternative Rock. Why? Audience A will always be Audience A, but Audience B craved something slightly more. Being exposed to Audience C's music, Audience B demanded MTV shift a little more to their will as consumers. This could be described as a shift in dominant genre. Sure, Madonna was still releasing things, but definitely took a backseat to Pearl Jam. This is also the era where mainstream began to push more aggressive music like Rage Against The Machine, or Korn. (I mean, aggressive by mainstream standarRAB)
Recently, we've noticed a shift back to the 80s pop first ideology. The only problem is, I don't believe this is a shift in dominant genre. What this is, actually, is a result of the invention of the Internet. I'd say that with the Internet, Audience B is becoming more specialized with an exposure to Audience C's music on a grander scale.
With Internet culture, Audience B has diversified. What was the very commercial grunge/alternative "rock" audience is now becoming too specialized to target. So, they're pushing forward Lady GaGa to appeal to Audience A, who want music that's specially accessibly, and can be forgotten in a week for something that's basically the same.
I don't think this makes "pop" the dominant genre. I think this is proof that in this day and age, genre itself is dissolving. If we're going for simply nurabers, than pop rules the day. However, I believe it's much weaker than before, it's just that other genres have split into allowing a more customized view on music per person.
Then again, maybe I'm just an optimist.