That's massive by the standards of modern democracy. Can you name any issue which more people have voted on? Far less people than that vote in most governments.
Yes I can. Here in Australia we've just held a non-compulsory national poll (technically a survey not a vote, although it's been debated in public as though it were a plebiscite) on making the legislative changes to permit same-sex marriage. The turnout is 78.5% of eligible voters, thereby exceeding the Brexit voter participation rate.
NB: The comparison may be confounded by local behavioural standards. Participation in this survey wasn't compulsory, but Australia has mandatory voting in most local and national elections. So there may be a prevailing culture of compliance. Nevertheless, a cursory review of the popular press suggests the turnout is substantially higher than that expected by most pundits.
People could be more passionate about the issue. With Brexit people didn't care as much. Im even doubtful it will really make much of a difference looking at it backwards 10 years from now.
NB: The comparison may be confounded by local behavioural standards. Participation in this survey wasn't compulsory, but Australia has mandatory voting in most local and national elections. So there may be a prevailing culture of compliance. Nevertheless, a cursory review of the popular press suggests the turnout is substantially higher than that expected by most pundits.