i'm assuming you calculated this by averaging the difference between one-handing and two-handing a longsword for comparing against the greatsword?
Yeah so, longsword is 4.5 average damage one-handed (6.5 with duelist style), longsword is 5.5 average damage two-handed, and the greatsword is 7 average damage.
I know a lot of people look at the maximum damage to see how effective their attacks are, but even in this case, the numbers aren't exactly that much different: 8 damage for one-handed longsword, 10 damage with duelist style, 10 damage two-handed vs. 12 damage greatsword.
It's not the weapon that's really doing the damage for melee characters, but the special abilities, fighting styles, feats, and their Strength score, so fiddly rulings like this don't really "balance" anything. It might effect your verisimilitude, but I put this in the same camp as forcing Clerics, who are intended to use shields to survive in melee with lower hit points so they can use their touch spells, who are
also supposed to smack things with a blunt instrument when conserving spell slots*, to take a (optional!) feat with Warcaster, just so they aren't stuck in this endless loop of sheathe weapon, cast spell, unsheathe weapon, melee attack, which wrecks my verisimilitude (because just think about what that would look like, in reality, with a guy readying and unreadying his weapon every 6 seconds), just so the Cleric can't make an opportunity attack every now and then.
*Sure, Xanathar's gave us
Toll the Dead, but from my experience with poison spray on my Yuan-Ti, give me an attack roll cantrip over a saving throw one any day. Especially on a Cleric, who has reasons to have ability scores other than a maxed-out Wisdom.