Acording to Scotweb's tartan lists, if that is Bruce Modern tartan, there are two 16 oz versions from Batley and Lochcarron, and four 13 oz versions from House of Edgar (2 different ones?), Batley, and Lochcarron. Between the 13 oz and 16 oz weights I would expect a moderate difference in sett size, but there may also be significant sett size differences even in similar weight fabric. If the plaid was bought from the same vendor who sold you the tartan for the kilt, and they are supposed to be the same weight, I would be seriously scratching my head wondering what went wrong.

But as others have said, if the tartan colors match well, sett size differences are probably not worth losing sleep over. Although, I rarely (?never?) wear a simple fly plaid so I may not be the person you want to ask.