I totally agree with you Joyscarer, its a bug bear of mine. You see people being almost railroaded into moving onto horses from ponies that they are just not capable of riding.
Personally I don't do big horses I'm 5'9" and ride a 14.3. And to be perfectly honest I went out looking for a 15h+ gelding and came back with a 14.3 coloured mare. My list of criteria was a hardy pony/horse who wouldn't mind not being ridden every day as without a menage or anysort of lighting available to me in the fields in the winter my riding is limited to the weekend. People still look down on me and say that I should have something bigger and with a bit more "pep" and my words back to them are phaaaaa why should I? I defy anyone to find a more level headed horse with such a big heart and so geniune right to the last, they don't know my horse, they don't know that she can have her moments but they also don't know that I could put an absolutely beginner on her back and she's going to look after them like they are the crown jewels.