That's not a Mass Market size. Mass market AKA "pocket book" is smaller with an actual text area slightly smaller than a 7″ 3:4 aspect ereader with no margins. Typically 4.25″ x 7″ with 6.9″ diagonal text fill.
The next common larger size has text similar in area to an 8″ 3:4 aspect ereader like the Sage.
An initial paperback print run is often Trade size which is anything between 5.5” x 8.5” to 6” x 9” (about 9″ text fill diagonal excluding headers and footers).
See
https://en.wikipedia.org/wiki/Paperback#Mass-market
Though it's not accurate as often the hardback and initial Trade paperback are not the same in the UK and there may not even be a hardback.
I've series where there are 4 different sizes of paper editions (Discworld is one example). Some people find that very annoying.