Have just compared an old style and new style medium - doesn't seem to be too much of a difference really but I'm sure I heard the new style were bigger fitting?
Will try one of these new flowery ones on Matthew tomorrow to see
His old style medium fitted from about 8 months til potty training a couple of months ago - he was chubby until walking and Evie looks like she'll be the same build. I bought medium flowery ones for Evie so they would last longer but I might have to buy another one in small so it fits now - they're lovely!