Originally Posted by Trakhak

Big rings on TT and triathlon bikes: that's a marginal-gains thing. It's not about muscling big gears.

The idea is that with a bigger big ring, you mostly use the sprockets toward the center of the cassette, rather than the smallest ones, on flat and rolling terrain.

The most important advantage is that the jumps between adjacent sprockets are smaller (in percentage terms) past the smallest sprockets, which facilitates staying close to your ideal cadence and power output

Also, the chain line is straighter, reducing the friction that results from transmitting power through a chain that's twisted sideways.

Finally, friction losses are greater with the chain on smaller sprockets than on larger ones.