Note though that there are two different storage locations, images and thumbnails take up disk space, while image metadata and other data (e.g. visits history) is kept in a database (which of course also takes disk space somewhere, but may be different). So when your hoster speaks of "storage capacity limits" you'd need to know which.

If it's database then likely removing different image sizes will not gain much other than maybe a few entries per image. It might as well be that you gathered a gazillion of visit details in the history. Check the database tables if something has an abnormal size. If it is the piwigo_history table (or if you're running [extension by grum] AStat already then use that to check the number of entries on its Tools tab)  you can install [extension by Eric] Prune History to prune data and enable Automatic Prune with a sensible value. If you're running Piwigo 2.9.x though then the size shouldn't be much too large as it has an autopurge functionality, see [Forum, post 170222 by flop25 in topic 28604] Automatism keeping history small

Those are the sizes that are offered to visitors. They aren't necessarily created (until a visitor requests them or unless you tell Piwigo to create all) but created on the fly and then cached, so may build up using disk space over time (you can take a look at the _data/i/upload/ directory). You can explicitly delete them (or selected sizes) under Admin -> Tools -> Maintenance -> Delete multiple size images.