Short answer: given everything else equal the ones that are too big will be much worse.

Now, long answer. To minimize chafing shorts should move as much as possible together with the body as a single unit, so that friction happens between the shorts and the saddle, and not between the body and the shorts. Obviously, to achieve this shorts can't be any loose, they should be skin tight. As I started cycling in 2017, I lost a lot of weight in the first few months, so experienced this first hand - as I was becoming skinnier, I needed to change shorts to a smaller size as they start to chafe.

"A bit too small" part is IMHO quite difficult to achieve because cycling shorts are usually very stretchable. I guess you mean something that doesn't rip apart right away and you can still wear, but it is stretched to its limit? My guess will be that you'll not experience excessive chafing but overall comfort may not be too great and shorts will wear much faster than usual - e.g. seams will start to rip apart etc. The thing is that it is pretty much impossible to be between sizes in cycling shorts because, as I already said, they stretch a lot, plus pretty much every manufacturer has a few lines of cycling clothing with different fit, plus clothes from different manufacturers have different fit! For example, DHB has at least 3 different fits: active, performance, professional - and the "same" size (e.g. M) will be very different between them. I mean you should definitely be able to find a size that fits you just right - sits snug but not too tight. Order a couple that should fit you according to the sizing chart, if some will not fit - return them.

Apart from this, construction of the shorts matters a lot for comfort: the way how chamois is attached to the shorts, type of seams (seems that are not flat will rub), fabric of the shorts and chamois, a pouch for the private parts (or lack of it on all cheap shorts). This last feature in particular makes a huge difference for me personally.