Editor Lucy Tuck said: 'The change of size entails a change of pace. Things have to be shorter and snappier. The larger format is perhaps starting to look slightly dated.'
TV and radio will be removed from the back of the book and put into a separate A5 supplement.
Fashion, celebrity interviews, food, home and living will continue to be the magazine's staple content.
Meanwhile, the Sunday Mirror has followed the raft of changes at its daily sister title (PRWeek, 19 April) by launching M Celebs.
Edited by Mel Brodie, it is an extension of the M brand, aimed at young women, which now runs four times a week across Mirror titles - as a newspaper section on Tuesday, M Health on Thursday, and M magazine, of which Brodie was formerly assistant editor, on Saturday.
Sunday Mirror editor Tina Weaver oversees M.