The recording resolution is generally based on the videos and content shared in the session. Based on that resolution, I would guess that you shared your desktop or something in that resolution, which in turn made the entire recording that resolution.
Unfortunately, this cannot be preset or configured currently.
I have been using Zoom for a few years now and have recorded locally. The recorded sessions I have with clients are often between 2 and 2.5 hours. I almost never share any content and never change the resolution on my monitors. Both monitors are set via Windows 10 to the recommended 1600 x 900 displays.
For these consistently 2 - 2.5 hour videos, the Zoom .mp4 recording sizes are either around 220kb, 440kb, or 1.4gb. So, I have no idea what is varying these files, but would love to know.
Even if Zoom told us specifically which elements/issues make them go 1+gb larger, that would be very helpful.