Wikipedia's page claims the "regular" was 62.5g and now 40g https://en.wikipedia.org/wiki/Mars_bar#Sizes
Since they downsized apparently in March, it is likely that the sellers on Amazon mentioning 51g very prominently are just selling off old stock.
alt Hacker News
Since they downsized apparently in March, it is likely that the sellers on Amazon mentioning 51g very prominently are just selling off old stock.