The regular economy seats are all very similar in pitch.
All three airlines also have some rows available with extra legroom. These seats can be purchased or are free to select for frequent flyers who have reached the status milestones in the FF programs.
United's extra legroom seats, called Economy Plus (E+), is the largest of the big three. Still, you'll see the regular economy section full, or nearly full, with significant amounts of E+ seats available. Those E+ seats are available to most of the passengers in regular economy but most are choosing to stay in the cheaper seats with less legroom.
With DL and AA's smaller extra legroom sections you may not see as much of a disparity as you do on UA.