I've been noticing these differences for a while, is there a way to reduce the size of your coins? I've tried sending coins to the same address only to increase the end size of the next tx and pay more fees. Thanks 
How so? If you were to select inputs and spend it to yourself, you would effectively be combining those inputs and that was what I was doing sometime ago. You can choose a small fee for those kind of transactions if you don't need the funds in the near future. Anyway, you can accelerate your transaction using ViaBTC's interface[1]. You just need a fee of 0.0001BTC/KB.[1] https://www.viabtc.com/tools/txaccelerator/
Don't use anything provided by ViaBTC. They are garbage.
Anyway, just request higher payments. The more inputs you get, the bigger the transaction fee, so request that payments are dealt monthly and not weekly. Bitmixer should look into that, I don't see why it has to be weekly and not monthly.
With segwit activating soon hopefully we get lightning network so we can end this nonsense.