What is a barrel saddle pad?

the horse i ride 23 year old arabian, is somewhat skinny, because she used to be neglected she now has enough weigh to ride and i have ridden her a few times. usually i use a foam support pad underneath then a saddle blanket. the saddle blankets always seem to big. so i just found this "barrel saddle pad" i liked how it was smaller in size. a href="http://www.chicksaddlery.com/page/CDS/PROD/1030/FBP623" rel="nofollow"http://www.chicksaddlery.com/page/CDS/PR…/a i was wondering if i could use this double fleece pad with a regular western pleasure/trail saddle. i would like to use this fleece one so she doesn't have to wear a support. will it fit the saddle?