edit: And I've seen interviewers pause at such titles and comment "are you sure you want to work here, we can't offer you as nice of a title." So it may count against you even.
This is a sector where people with no relevant formal education and three years on-the-job experience sometimes get called senior.
At the end of the day, job titles are mostly for getting past recruiters. Interviewers (if they're any good) will judge you on your actual experience and capabilities.
I'd say the extent of actual impact to business relative to the entirety of the business operations probably remains roughly the same between two people with the same title at a large company and a small company. However, the potential loss (or perhaps gain) suffered from poor (or great) decision-making is likely much smaller on an absolute scale at a start-up, although roughly the same on a relative scale.