File removed but project still very large


recently I permanently removed some files from my gitlab project (see there for example). Locally my project shrinked down to about 20Mb (one developer added about 700Mb of unwanted files by mistake). Although I pushed the changes and the files are not anymore present in the history of my commits on the gitlab repository, the size of my repository as showed in the “home” of my project on the gitlab website is still of 790Mb. So the files must still be present somewhere. Do you know a way to remove the files from the gitlab repository?

Thank you in advance for your answer.

Best regards.