Its not so much the height as just the colour, Dragoons usually got inferior horses, the Swedes were likely an exception, but the horses regiments got their pick of the darker colours so Dragoons would be just as good horses but a more mixed bag of colours. My understanding is its why the Scots greys were greys, the horses regiments wanted dark horses and the scots greys got decent horseflesh but in paler colours. Many of the Northern European armies subscribed to this odd idea it seems. Good news for the dragoons as they got better horsesflesh.