My last trip I knew my cell coverage would be too poor to stream but I was also running out of space on my phone, so I re-encoded my stuff to 360p, low ass bitrate.
It was a bit blocky for some fast paced action scenes. For sitcoms? Totally fine.

Is it worth 4 times the file size

For a TV or a Tablet, sure.
For a phone? Meh, unless you have a ginormous phone (mine is 6.67") I wouldn’t bother

Depends on the content: something like western animation, or something like documentaries and news reports, do not really get anything worthy going past 720p and sometimes even that is too much. And even the most blink-and-you-miss-it-scenes series on TV nowadays do not really justify spending 1080p for the entire duration of the episode.

Depends on the playback: If you are only going to watch on your machine / your own phone, you can go with whichever but if you intend to watch on a TV set for example, or if just intend to share the file to other machines in general, x264 has much wider compatibility and stresses the hardware far less.

It depends on how you intend to view, and the source.

DVD source? No reason to go past 720 as that’s all they do.

Blu-ray can do a lot more.

Interestingly on a 65" 4k TV, I find most 720 sources to be fine. I did just watch Blithe Spirit from DVD and it was awful. Lots of blurriness. Someone really screwed up the encoding on that one.

As others have said, test your encoding. I’ve generally found with handbrake that movies converted with a Quality level 19 (H264 MKV container), reduce about 70%+. So a 4 GB video often reduces to 1 GB, and you can’t see the difference on the screen.

Some you can, but those are movies with a 16:9 aspect ratio and letterboxing built into the video so DVD players show them correctly. Since that loses some pixels up front, they require using higher quality levels to prevent visual lossiness.

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.

For watching on a phone, probably not.

For watching on a real TV, definitely.

If you’re downloading for a permanent collection, I’d get the 1080p version. If you’re downloading temporarily to keep on your phone, then save the space.

Why not compare 720p 265 to 1080p 265? I don’t get anything in 264 these days, 265 support is ubiquitous and the performance is so much better.

This is a matter of personal preference, not something where a consensus opinion helps. Where you’re at, it sounds like 1080p x265 isn’t worth it. Would it become worthwhile to you if you were using it on a different device? With a different set of conditions (like lower battery drain)? If a small change can make it acceptable to you, it’s a hangup, not a standard.

Are you watching it at 200x zoom?