How do I convert measurements from cup?

If a recipe is telling me to use 1 cup brocolli, 1 cup cauliflower and 1 cup spinach how do I convert that to grams or ounces? I have found info telling me that a cup is equivelent to 8oz but surely you would get more so much spinach if you weighed out 8oz but not as much cauloiflower as this weighs more! Does a cup mean how much you fit into a cup size? Im really confused and need an answer to this before lunchtime!! Help!!