I also don't have landline ISP, and use 3G dataplan/month. I have several computers running MIP and also am running out af data after a few weeks because of MIP.

I think the big files (*.1 and *.2) can "carry" a lot of tasks within, but then they are distributed to several computers, creating more datatraffic.

I think let boinc communicate only once per few days might fetch you more tasks per big files but am not sure.
If your using several devices in your local network a https caching proxy server also works.
Myself am planning to make a proxy server on my local tethered network with ssl-bump and use it for boinc. Certificates are in C:\Program Files\BOINC\ca-bundle.crt but have not succeeded yet.

The initial download is going to be bigger than the regular work units. Maybe that's what you measured.
Also, it looks like you are trying to get a five year badge in each project. Are you only running MIP? Each active work unit significantly slows the others down, creating a lot less overall progress.
You may find that mixing in a few more projects is easier on your data and computes more science (as measured in points per day).
If you are on Windows, I strongly opting into everything except Ebola, Children's Cancer, and Zika. If you are on Linux, I recommend specializing in those three.