A gilet can be either. If you are racing or riding with a little more purpose or simply like the style more, a tighter-fitting, racier gilet is going be more suited to your needs. It won't flap and create drag and you may just feel a little bit more agile on the bike.
If you are riding off-road, taking it a bit easier or just prefer a more relaxed fit to layer up more underneath size up or choose a more relaxed gilet. The choice is yours. A lot of manufacturers will also indicate the fit of a piece in their product descriptions so look for this or ask if you want to know more about a particular garment's cut.