If you, as a user, feel that the site is sluggish, unresponsive or difficult to use because it feels too crowded or has too many features, you might judge that site to feel bloated.
In a more technical sense, bloated might refer to very large (or poorly optimized) files, either javascript or static assets, required to run the thing, causing it to load slowly (i.e. at a speed that the user feels is "slowly").
So maybe a good summary might be: "does too much stuff and/or poorly optimized/coded"
Again, not exactly a tight definition, but UIs are inherently characterized by the perception of their users.