A lot of gyms offer at least two sizes of weightlifting bars: one with a larger grip intended for men, and a smaller-grip one made for women. The thicker the bar, the more surface area your hand has to cover, making it harder to hold on to. So if you’re going for performance and chasing a PR, your hands may end up giving out before your muscles have a chance to show what they’re capable of.
A skinnier barbell will stay nestled properly in the palm of your hand with less chance of slipping—especially if you add chalk to the mix. This allows you to really squeeze the snot out of the bar, maximizing your power. Plus, if you’re deadlifting, it may prevent you from resorting to an unhealthy mixed grip in an attempt to compensate for hand strength.
There are times when thicker is better. For instance, large barbells or handles can be a training tool to explicitly work on your grip strength and for building forearm muscles. If you’re doing pressing movements, such as bench or shoulder presses, or dips on parallette bars, a larger circumference will help disperse the load across your joints—important especially if you suffer from achy shoulders or elbows.
Note the weight difference between the larger and smaller bars. (A trainer can tell you the poundage if it’s not marked.) One additional upside of the smaller ones? You get to load on more plates.