I think it depends on what IUD you have. Some release hormones (same as the pill) and this may cause breast enlargement. The actual size of the enlargement varies from woman to woman but I don't think it results in huge boobs ;o)
Thinking about it yes I think it is true. I had mine put in 18 months ago and I have gone up a cup size. They seem to keep getting bigger and I couldnt think why.
I'm going to have a look for more information about it.