That's not necessarily true. I have ears that generally need different sizes (one more small/medium, the other medium/large), and know there are more out there.I suggest offering the eartips in packs of individual sizes: Small OR Medium OR Large.
Only one size is going to fit correctly. That leaves 4 pairs out of the 6 that are useless unless someone is truly in between sizes.
It's fine if FiiO wants to sell the S-M-L pack as a sampler for buyers who are unsure what they'll need. But I know I need Large -- why should I bother with 4 pairs of extra tips?
For the people that got this, I read that it had good isolation.... Does it also NOT leak sound easily?