I think niche market / big market is always subjective and very poorly defined in general leading to confusion. When chatting with a VC, "niche" probably means "not worth investing into, too small" in their simplified language even though you could mean simply that it's a subsection of some huge industry which is in itself huge. Like first person shooters in the game industry.

Maybe a weird example: most fursuit builders also take mascot commissions from non-furries. They target a small niche, but also a big market.

A niche of a giant can still be considered “big”.