I'm interviewing for a position in a small startup, currently 6 persons, expanding to 9. The title of the position is COO (Chief Operations Officer). Having only worked in large companies (400 is the smallest previous company), it sounds to me quite inflated
My concern is that after this I will state I'm COO, but for a 9 person company, sounds like a joke. Should I be concerned? What are your thoughts?

Everyone agrees... companies come in all sizes...
...and all of them need a CEO. If the nature of the company is such that a CEO must maintain her focus on the customers (recommended) and doesn't have capacity to adequately manage the internal operations of the company, then it makes perfect sense to me.
Take the title and focus your attention on what really matters, results and relationships. CONGRATULATIONS!
--Michael
Compare to similar titles in large companies
Compare to similar titles in large companies. At startups and other small companies, folks wear many hats. This wears out a lot of soloist businesspeople, who have to wear all of them!!
Would this small-company role encompass "COO"-type duties and below if the company were a hundred times bigger? If so, no title inflation, just recognition that this necessary role is indeed filled by someone other than the CEO.
As the company grows, you'll be called on to carve off portions of your growing job into new positions filled by others. After all, this is exactly what the CEO is doing by splitting off the "COO" part of his job and hiring you into it! You might start as "Head of Operations" (as well as all the rest of Operations!) and end up as "Head of Operations" with several hundred people ... operating.
Joseph (old used "CEO/CFO/CMO/COO/C*O" aka sole proprietor :-)
Thanks
The responsibilities are similar to a COO role in a larger organization, but the scale of organization size and budgets is just smaller.
Thanks much for the comments!