I have never invited the whole class to a party and never intend to unless I win the lottery or find a play barn has appeared overnight in our garden. 2/3 of the class is the limit for me, so the uninvited are in a large groups as well. This is partly due to cost and partly due to convenience.
I really resent getting caught up in a spiral of huge expensive 30 children parties at special venues just to repay invites. All that commercial and social pressure. I always make sure my son gives a good birthday gift to the host and that is were the obligation ends for us.
Mind you, I do feel ever grateful to the heroic parents who give whole class parties especially those generous souls who do it year after year. I sometimes invite host children on playdates if I cannot invite them to parties.
As my oldest son moved up to year 5 and 6 in primary, sadly there were one or two children who automatically stopped getting invites to whole class parties. They had a reputation for misbehaving (and they could be very disrespectful in party situations and word got around) so after one parent excluded them from an all class party, all the other parents followed suit