In order to get the dialogue going to answer this question properly you are going to need to join GoY Guest. As a starting point what sort of soil are you growing them in? Was it heavily composted or manured? Watering did you? If so how regularly? Oh and what is a 'nice size' swedes in the shops are, in my opinion far too big much better to have something the size of a large jaffa orange than a basket ball.

Are you sowing them early enough to give them time to swell?