The account size is calculated on the ledger entries created in a rolling 2 month period. If the system prompted you to purchase a subscription 12 months ago, your account would have been graded as L or XL. Your usage (at least in terms of the number of ledger entries created) has likely dropped and so has your account size.
As your account is now a M, there’s no need to renew the subscription unless you wanted to optionally. If your account is regraded as L or XL again, then the account would require a subscription.
I’ve included a few links to guides below which may help: