You have to match the sts/inch of the yarn with the required sts/inch of the pattern or the size of the garment will be wrong. If your DK yarn is 5.5 sts/inch on probably a size 5 to 7 needle then your pattern has to be sized for approx. 5-6 sts/inch. You can fiddle around a little with needle sizes to get the correct sts/inch for your yarn to match a pattern, but not too much. If your sweater pattern is for a bulky yarn like 3 sts/inch on a size 10 needle, there is no way you are going to be able to make that sweater with your DK yarn. Always check the sts/inch that a pattern requires or your sizing is going to be wrong.

Ex:- a sweater pattern, meant for bulky yarn, that says 3 sts/inch and you cast on 60 sts is going to be 20" wide. If you use dk yarn that is 6 sts/inch with that same pattern your sweater is going to be 10" wide!