In period, it was not that snug. This was in an era when you could get a dozen people into town in a 2CV and the majority of adults smoked. People were a lot leaner then, and considerably shorter.
In 1954, in the UK, the average male was 5'7" and 11 stone, 6 lbs, which equates to quite a high BMI of 25 (18.5 - 24.9 is healthy BMI). Nowadays there are very few men in the UK with that weight. Let's ignore women from this because women were second class citizens at the time. Men are on average 2 stone heavier.
But average weight is just average, nowadays with 2/3 of UK adults overweight, there are far more people at the top end of the Bell Curve, at double the average weight people had on the 1950s. So half of today's population would not fit in the seats.
When you look at old European or Japanese cars such as the MINI or old FIATs, bear this in mind. Those cars might not have ever been spacious, but, in period, they were not cramped. Similarly, Concorde was never spacious, but it was not a very snug ride until obesity got the better of us.