Why are my file sizes so different when recording in full sunlight as opposed to overcast?

My settings are the same, 1080, auto.  A 3-minute video in less light is nearly twice as large -- and becomes choppy/smeary when played on a older computer with a slower processor.  On the same computer, the 3-minute full sunlight video plays without issue and -- as a matter-of-fact -- it looks fantastic.

Can anyone explain why this happens?  Or does my MA have a problem?  

I think I found an answer.  I set my MA to 1080 30-frames per second --  but it changed, all by itself, to 60-frames-per-second.  My old desktop computer does fine with 30 but struggles with 60.

Anyone know why this change to 60 fps happened?