of cause it's mvcc and can be vacuumed by autovacuum or manually.
but 10's of GB sounds wrong for me, not sure how many tables you have, but it's just too much.
anyway you can vacuum tables more aggressively on system level, or manually vacuum the stats table.
see my test with 1000 tables 10K rows each.
select pg_size_pretty(pg_total_relation_size('pg_statistic'::regclass));
-- 504 kB
do $$ declare i int4;
begin
for i in 1..1000
loop
execute 'create table t'||i||' as select generate_series(1, 10000)';
execute 'analyze t'||i;
execute 'drop table t'||i;
end loop;
end $$;
select pg_size_pretty(pg_total_relation_size('pg_statistic'::regclass));
-- 888 kB
vacuum pg_statistic;
select pg_size_pretty(pg_total_relation_size('pg_statistic'::regclass));
-- 504 kB