And probably at it's greatest on long haul flights....

Where (fr'instance) in:

Coach (aka Economy) the seats are about 17.5-18.5" wide and the seat [I]pitch[I] ranges from 31 to 34" ( some airlines offer more width and/or pitch in "Economy Plus/Premium Economy" options)

Business OTOH can have a traditional seat with a width over 21" and a pitch of anything up to 60".. or, with newer products, the seat turns into an angled flat bed (head slightly higher than feet) or a truly "flat bed"...

First class these days on international flights can be anything from a very large conventional seat, to a sealed off mini cabin!!!

Have a look at jetphotos.com, and select any large passenger aircraft type and "Cabin interiors"... That will show you the HUGE differences that exist!

(Exceptions apply of course, both up and down)

Seat pitch is the distance from any point on one seat to the SAME point on the one in front/behind....

BIG difference in meals etc as well......