Like consentrated sugar, natural sugar, but still sugar.One raisin has the same number of calories as one grape, but all the water has been taken out. However, one cup of raisins is not the same as one cup of grapes. You will get far fewer individual grapes in one cup then individual raisins. Hence raisins are a calories dense food for its size.
As for me I could easily eat a cup of grapes but not manage to eat one cup of raisins, I'd feel sick!