The lid on my 6.5G primary bucket is loose as well. Although it fits on the bucket, its not tight at all. I can literally spin it around the rim of the bucket with one finger. The lid says "ROPAK" on top of it and came with the Brewer's Best kit.
However, I didn't have any issues with the fermentation with my first batch. The FG was right at the target after 4 days. The lid being loose and thus not air tight, I did not see any bubbling activity on my airlock. So that concerned me initially, until I used a hydrometer to check the SG to see if the fermentation was going on well.
So all being said, loose lid shouldn't worry anyone. Yeast will do what its supposed to do.