Why does my iTunes add more memory to my iPhone?

I just had a 2 MBs left to use on my iPhone so I deleted some apps and ended up having about 800 MBs available to use. I sync it once and immediately its cut in half to 400 and a couple times letter I'm back to where I started and that's pretty frustrating considering I've added 10 MBs maybe total since that time. Does anyone know why that is happening? I thought it was genius play list but I turned that off and still nothing. Any help would be greatly appreciated.

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