When you are making a sample garment, of course the goal is to get the fit just right. It is pretty rare to get the fit perfect on the first sample, though, even if you have already sampled the style in 3D. Knowing this, you can strategically draft and make your samples to make it easy to dial in the fit.
If the sample is too small for the fit model, it will be really hard to accurately assess the overall fit of the garment. It is also tricky to measure how much too small something is unless you actually cut the garment open and pin in a new panel of fabric and measure the gap between the original pieces. If one area of the garment is too small, that might throw off the fit of the rest of the garment even if the rest would otherwise fit fine. The tight or too-small area prevents the garment from draping naturally on the body. If an area is so small that it can’t even be worn properly, it is usually not worth even fitting the rest of the garment. It is best to start over with a new sample.
The garment being too big is less of an issue. It is much easier to dial in the fit on a garment that is slightly too big than one that is too small. Pin in the areas that are too big and measure how much you’ve pinned out. You can still assess the drape and balance of the garment on the fit model when it is too big.
It is tricky to get the fit just right on the first sample. When spec’ing or creating fit samples, it is better to err on the side of too big than too small. This way, you are making it easy to measure the needed changes accurately and get the fit perfect on the next sample.