Font size won't enlarge in iBooks on iPad Mini

I used to be able (up to 11 December) to enlarge the font size in iBooks so that I can read in bed without glasses or contacts. Since 12 December, that function no longer works - it enlarges to about 9 lines per page, but it used to be much larger (about 5 lines per page). I spent 90 minutes on the phone to Apple Support, who had no idea why or what was happening, and basically said "the developers have done that in the latest update, sorry" but this is the main reason I bought the iPad Mini, so now it's pretty much useless. Any suggestions/solutions?