The average size of Glaucous is certainly larger than Herring, but there are some small ones that are similar in size. They are almost always ornerier than Herring.
Just to be sure there was no Herring Gull component in this bird: Did you see any dark feather tips on it?
The reason I ask is that many years ago I saw a similar “dominant” white gull at Revere Beach, but it had some slightly dark smudges on the outer primaries, so I was sure it was NOT a Glaucous. I couldn’t figure it out, so I sent the description around to various folks and one person , Davis Finch (legendary old-school birder, lives in NH) wrote back that it sounded like a hybrid Herring x Glaucous. And the only one like it he had ever seen was the orneriest gull he knew! That particular hybrid is now known as a Nelson’s Gull – no idea who Nelson was. Needless to say, I’ve been looking for another Nelson’s ever since.