I agree with what snippy has summarized.

In the majority of movie/music mixes, the average sound levels coming from the surround channels are significantly lower than front channels (except for those relatively few music recordings that sound like you are sitting right in the middle of the band players). This is because, in general, movie or music directors don't want to detract your attention too much from the front.

That said, the surround channels may still contain occasional peak sound levels that are as loud as the peaks in the front channel. So, you don't want to skimp too much on surround speakers.