Firstly a big welcome Abbie...as Issy says it really can vary. I am just wondering is he an English Cocker Spaniel breed in the UK with Kennel Club papers etc. - not that my boy has papers
(rescue cocker)
My friend in the States had two American cocker spaniels and I can honestly say that they were a quite a few pounds heavier than my two English cockers, in fact I honestly think I wouldn't want to attempt to sling them under my arm.
Also you do find some larger frame cockers my MIL's dog was a lot bigger than my present one. A picture would help.
I too wouldn't worry too much, apart from your expectations etc of what you where told when buying, a healthy loving dog is the main thing.