The frame size setting to 1280x853 may be due to starting with a frame size with a different aspect ratio. In the Export Settings Video tab, you could try clicking Match Source button (which should set to 1920x1080 if that was your source clips) and then type in 1280 for Width. If that still results in 1280x853, you can click the chain link icon to the right of Width & Height (tooltip is "Select to maintain the frame aspect ratio while resizing.") to no longer force maintaining frame size ratio and then type in 1280 for Width and 720 for Height.
It's a challenging balance game to create Proxies that are very small in size, yet have good quality and good performance. H.264 can get you good quality and small frame size, but the compression required to do so is not very performant. Going with something like QuickTime GoPro CineForm - Mac & Windows, Apple ProRes 422 (Proxy) - Mac or DNxHR/HD QuickTime or MXF - Mac & Windows can get you good quality and performance, but will be larger than H.264 clips.
I would recommend some investigation using CineForm or ProRes or DNxHR with smaller frame sizes to find the best balance for your workflow. These codecs have good quality, so you might be able to go smaller than 1280x720 frame size to get the file size smaller, keep good performance with hopefully acceptable quality.
It is that or get more storage to accommodate the CineForm/ProRes/DnxHR Proxies.
Hope this helps.
Thanks for taking the time to answer this, I appreciate it.
I found that when I converted to a .mov file rather than using mp4 before making the proxy, it didn't make them a different width. I'm not sure if that's the right answer, but it seems to work.