I think you're still a bit too hung up about the size of the companies and not the nature of the trading partnership. It also sounds very much as if you're talking about using a channel model for the business. Now that's a good method to use, but reading a bit between the lines here you're not sure where this customer should sit in the chain.
Probably taking a look at what they do and who they service will give the strongest clues. You could also just ask them outright - nothing worse than going down one track only to find they have been thinking along different lines about the business relationship.
You're absolutely right to be concerned with ensuring that the smaller company doesn't have an agreement that kills it though, but that's juts as much to do with the terms of the agreement rather than the nature of it.