I‘m guessing the API doesn’t return that info and it‘s different from instance to instance. Other clients also don’t show it. I noticed that it is also a question of dimensions, not just file size. But a terrible experience all around. I gave up and been using Imgur ever since.
I can’t remember which instance, but I did see a post from the admin of an instance who said they had to limit basically anything bigger than the avatar because they didn’t want to be a host for questionable material. I imagine a lot of instances have similar “no image hosting” policy.