Loosen tension bolt.Get a big pot of water,bring to a boil,drop seat in for about 2 min.Get a heat gun,not a hair drier,and shrink the seat until it's as tight as you want.If it's not tight enough,add a few drops of liquid dish detergent,NOT SOAP.Repeat.

Let dry in the sun.Seat will now be as hard as a rock and ready for a new break in period.Depending on how it was tanned,will decide on how long it will last.

Shrinking leather work pretty much the same as felting wool.The same type of things are happening.Hot water expands the fibers and lets them slip past each other.Detergent lets them slip more.

You can't keep going like on wool,turn your sweater into a thick one for your kid,but it will shrink alot!

You can tan leather many different ways,vegetables and tree bark(tannic acid),urine(ammonia),brains,ect.

Being that Brooks is a commercial venture,I'm guessing they use tannin(the best)(they may use urine,2nd best),so the hot water trick should work.