See, there’s that ignorance again.
A browser is not just a simple HTML viewer anymore.
A modern browser has to handle extremely complex code, combining HTML, JavaScript, CSS, and more, and at the same time do it very quickly. It has to store data, remember states, cache DOM structures, etc. Opera in particular was designed to do a lot of caching (which is why it is probably the only browser that allows you to go back instantly most of the time).
And then there’s the fact that browsers like Opera store closed tabs, and more.
You are completely ignorant of how memory usage in browser works. That is not an attack. It is a statement of fact.