It’s because it’s an electron app. So in addition to the chat app itself, it also includes a full Chromium runtime. Worse still, the Electron architecture doesn’t really lend itself towards reusing electron itself; this means you might have several copies of the same version of electron on your machine for various apps.

People complain about the sizes of things like flatpaks and snaps, but tbh the whole architecture of applications is like this these days. Ironically, flatpaks and snaps could help with this because their formats can work decently with filesystem level deduplication.