6.5mb audio input, 36mb audio output! where is the extra 30mb coming from?

I've created a simple audio file (mps) made from 4 separate files and a sound effect. The 5 files are all quite small:

File 1: 2.34mb

File 2: 2.58mb

File 3: 859kb

File 4: 580kb

File 5: 77kb

Total: 6.436mb

So why when I put these together in Premier Pro is the output 35mb!

Any ideas, is it me? should I be doing something else, I just don't get why the output is so much bigger. 

And to top it off, there is also some minor distortion which isn't on the original.