Japanese is generally more compressed than English. For instance, consider "dragon" vs "龍". The exception is when polite/formal language is being used, in which case it can get more lengthy. Instructions and descriptions would generally use more polite language, where navigational elements would be more pithy.

If I was to offer a theory on why the Japanese tolerate more text, I would say it's related to higher "skimability". It's easier to pick out symbols within a block of text and get a general feel for what's there vs. words in western alphabets. If this is the case, we would also expect Chinese websites to be similar, and I think that's the case.

Or, it could just be a design trend. For instance, usage of the color pink is much more prevalent in Japanese design.