Having small video files from iphone videos, either compressing large ones or recording them so they are small in the first place
Apple makes a pro app for Mac, Compressor, that can handle this task, but might be overkill. It can handle batch jobs etc.

There is also Roxio Toast which is a DVD burning utility but has a nice suite of tools for video conversion. You could use this to make DVDs or Blu Rays of all those vids and then get them off your system entirely. A Blu Ray burner is around $100-200 these days. 

I use Panasonic 50-year-archival BD-R media, which was about $100 for 25 discs that hold 25 GB each whenI bought it last year. I burn all my HD videos to those, then delete it off my computer entirely. Then to watch it just throw that disc into your Sony PS4 or XBOX One, or Blu Ray Player attached to your TV.

Ideally you could do both: archive the big HD files to BD-R for backup purposes and posterity, and keep highly compressed files on the Mac for the occasional viewing.