What could you possibly be using all that space for?
Do you have your data written in with your code?
You probably need to do a total re-structure.

Thanks for your reply.

I'm not sure for the space.
But we have four different visualization methods (around ten classes), each visualization has around 2000 objects with animation, attributes.
And we have external data read into the attributes of the objects.

260 MB isn't that much by modern standards. Does the browser has an issue with this memory usage? By default, Processing asks as least 64 MB and allows memory to go up to 1024 MB (but once can adjust these numbers).

Memory can be consumed by fonts you have loaded (you can reduce their charset, perhaps), images you loaded, sounds you play, less likely textual data (unless you parse megabytes of text!). If you create lot of objects (a typical case is sketches with lot of items moving around), it can take up some memory too, depending on the number of fields of the classes.
You also have to take care of memory leaks: creating objects, no longer using them, but still referencing them in some collection. If you create PImages or PGraphics, they can be memory hungry too.
The libraries you can use can also create objects, it is harder to control the process.

There are lot of factors on memory consumption, we cannot go beyond general advices without knowing what your sketch does exactly.

Thanks for your reply.

I'd better look into the codes in details and try to re-design some classes. Some of them are in a little bit mess now.