Sorry!
Most bra companies in the UK go as follows: A,B,C,D,DD,E,F,FF,G,GG,H,HH....... so if you have a 10" difference, you'd be around a GG, a 9" would be a G
M&S miss out the FF. So in their bra-alphabet, a 10" difference means an H cup. So the 32G you tried on in M&S is only for an 8" difference rather than 9" and is equal to everyone else's 32FF.
In terms of cup volume, cup sizes are not equivalent between bands. What actually happens is that to maintain the same volume of breast space, as you go down a band you go up a cup. So, a 30G, 32FF and 34F would all be able to hold the same volume of tittage.
Add to this the fact that M&S bras tend to run small in the band (loads of reports of this in the last 6 months), and the 32G you tried on was probably more like a 30F in one of the decent brands like Cleo, Freya, Bravissimo, Panache etc.
Does that make more sense?