Just compare the economic growth per capita of Norway (not in EU) with Sweden (in EU). It's at best an exaggeration that leaving the EU is going to have a significant deleterious impact on the UK.
It's not quite an equivalent comparison, though; Norway is essentially a Middle East petrostate relocated to the North Sea.
Norway's arrangement is much more comprehensive than the UK's, though.
But honestly, I’m not an economist and haven’t read a great deal about the projections. And many Brits will accept any difficulties in exchange for increased control over their own political destiny.