Thanks for asking, Mike,
I will try to guide you though this thought, each part will build up on the next.
I talked about photos, not renderings, hence the link.
When I use large euquirectangulars, then to retrieve a still or a video from it. Example, around 17 years ago, I had to produce an 20 minute architectural visualization; Deadline a month later, with six different objects (interior viz). So, I rendered one equirectagular (for each room) and placed it on a Sky Object (sphere) and from there I made a camera animation (Camera stayed on the center!), like watching an equirectangular-still with a head mounted display these days. It is fast and clean.
An Equirectangular is typically 2:1 , so only half high, not square. Keep in mind that the AA doesn’t need to be that high if the resolution is higher than needed, when you consider to down-sample. The results from photos are so much cleaner that way. No weird sharpening, which destroys quality if done globally. Note that renderings are typically clean and without any noise, but many techniques introduce noise, e.g., Monte Carlo.
When you do video, you could create a high res equirectangular and only have the animated parts in 3D in the scene, the rendering would be then just the those parts and merged later. This, again is just a suggestion, not meant as “best or only” practice.
When I go to
http://store.kolor.com/virtual-tour/panotour-pro.html
You will see the options to mix still and motion parts seamlessly. Example here: the TV in the clip has a movie, the rest seems to be based on a still. So, one time loading of the “environment” and streaming the little partial parts (animation) seams to be so much less “data”. What you see in the initial example of your post, might be only partially animated. Of course I don’t know for sure how they did it.
However, I hope I was able to share some thoughts and examples. In your example link, I did not found any “three.js” mentioning, which is another option, and usually created via Unreal or Unity as authoring tool. Just to be clear, I’m not a web-coder, and haven’t done any web stuff since decades nearly, so I’m not savvy here anymore at all. ;o)
Analyse your ideas, find the best way and options and deliver based on what is possible. To just dumb an 8 by 4 or even 8 by 8K file is certainly no idea. I agree.
All the best