Bitrate and resolution being equal, 264 is gonna be about half the size of 265. It’s an older codec, so it decodes easier on older hardware (like 10 or more years old). If you have anything recent-ish, 265 will save you space… all other things being equal.

Because 264 saved less space, people used lower bitrates. With 265, they’re using higher bitrates because storage devices have gotten bigger. Blu-ray rips that were 4-5GB in 264 are like 2-3GB in 265 and look better.

With 720p you’re reducing the resolution so that takes down the size too.

It’s really hard to say if you have no control over the encodes. Learn handbrake (honestly it’s not hard, handbrake intermediate users know like 90-95% what handbrake experts know, it’s a very easy and straightforward tool to use) and take control of your encodes. Tweak the settings as you like 5-10 seconds at a time until you find a setting that meets your size and performance needs, then save it as a preset and encode more stuff with those settings.