That's a good question, actually. "Fat Spring Bars" is an affectionate term for the large diameter spring bars included with Seiko Diver's watches. They are thicker than standard spring bars (2.45mm vs. a more standard range of 1.30mm to 1.08mm for most spring bars) providing more security over the long term compared to thinner spring bars. They are flange-less spring bars, so for cases without holes drilled through the lugs, it can be somewhat difficult to remove them for frequent strap changes (and impossible for certain bracelets that don't have a cut-out provided to access the end of the spring bar). But that is true of any flange-less spring bar, not just fat spring bars, and diver's watches aren't meant to undergo frequent strap changes- they are tools meant to be worn until the strap nears the point of failure, at which point the strap and spring bars are replaced. How many of us have worn out a rubber Seiko diver's strap?