[I have no idea how wide Ian's rim and tyre are. I assume he knows better than to fit a really wide tyre on a really narrow rim. These remarks are further to what Jim said.]
Originally the ERTRO recommendations for rim widths to match tyre widths was that rim width should be no less than 40% of tyre width, which is the same as saying the tyre could up to a maximum of 2.5 multiples of rim width. Then the idiot manufacturers, who were too slack to make wider rims to match the new Schwalbe wide tires, screeched to high heavens that their business was being undermined and ERTRO, which is a trade body, cravenly caved in and "authorized" the existing narrow rims for balloon tyres. It's a disaster that sooner or later will hurt a cyclist badly when a correctly inflated but too wide tyre splits a rim at speed.
Many rim manufacturers cheat and publish the rim's outside width with the implication that that is the rim's official width. It's a lie. The ERTRO width of the rim is taken across the bead retainers on the inside of the rim. Thus a 25mm rim, inside, correct measurement, is usually about 32mm wide on the outside. This is the narrowest rim on which a 60mm Big Apple should be fitted. Wider still is better.
Sensible cyclist will follow the old rule before ERTRO chickened out before undue influence:
MAXIMUM TYRE WIDTH IS NO MORE THAN 2.5 TIME INNER RIM WIDTH
as measured inside the rim across the bead retainers, not the outside of the rim
