I was just thinking about this issue again - specifically the question of space on public transport - and it occurred to me that I have different expectations of what is 'normal' depending on the circumstances.
In the case I mentioned previously, the particular coach I was travelling on could usually be guaranteed to be quite empty, and had already passed the stop at which most people normally got on, so in these circumstances having someone sit directly in front of me seemed odd. On the local bus home from work, on the other hand, which would usually be completely full, I would expect to have someone sit next to me, or have to sit next to someone myself, and wouldn't mind at all, provided they didn't actually sit on me.
In the same way, when I'm standing at the bus stop or in a shop, I expect the next person in line to keep a reasonable distance from me, otherwise I feel quite uncomfortable. But when I go on the London Underground at rush-hour that same person could be jammed right up against me, and it wouldn't faze me at all.
So my personal space bubble clearly has the capacity to expand and contract in different circumstances. Interesting, isn't it.
(I'd also like to add that I'm another person who dislikes having someone in the cubicle next to me when I am on the toilet. Rationally I know it doesn't matter in the least, and the other person probably isn't paying the slightest bit of attention to me, but it still bugs me.)