Once I have made a recipe a couple of times, I tend to follow my instincts rather than strict portion sizes.
When following a new recipe, however, I have often wondered about how an ingredient's weight should be interpreted.
For example, if a recipe calls for 500g of pumpkin, diced into 1cm cubes, does convention expect me to use a 500g cut of pumpkin, that I then de-seed and dice, or 500g of 1cm pumpkin cubes pumpkins?
In your example, it means 500g pumpkin before preparing.
In general, you might see this two ways:
The first option refers to the weight before prepping while the second refers to the weight after prepping. In general, the second one is far more exact for the actual recipe while the first one is more exact for shopping. I assume ease of shopping plus presumption of yield is why the first one is so often used, although in high end cook books (Grant Achatz, Thomas Kellar, etc) the weights are often given post prep for precision.
You will see the same thing for volume measurements (i.e. one cup of nuts chopped vs one cup of chopped nuts).
When I started cooking seriously I usually worked in terns of prepaed weights. However, my first thought (as a one-time scientist) is that although the recipe writer is may be lazy by not being exact in the specifications, I still take any recipe is just a starting point, and as I cook more I tend to keep checking and tweaking the recipe to try to get the right texture or taste according to the particular ingredients and the time of year. I have learned from my artistic partner who works in inexact units of bits, spashes , pinches, sploshes, and - especislly annoying - just enough ofs - until she's happy.
