Huh...first thing I thought was that she said that because the smaller a pumpkin is, the sweeter it would be. Kind of the same idea with any squash, really, like zucchini - you want to use those small because if you let them get too big they get woody and are a little bitter tasting. But I googled and couldn't find anything that said you SHOULDN'T use a big pumpkin to cook with. I found this on Southern Living's web site:
"Though canned pumpkin is usually used for pumpkin desserts and soups year-round, fresh cooked pumpkin can be puréed and used in any recipe calling for pumpkin. Smaller pumpkins are best for cooking, yielding sweeter and more tender flesh than the very large pumpkins.
A 5-pound pumpkin will yield about 4 1/2 cups of mashed, cooked pumpkin. One can of pumpkin, 15 to 16 ounces, yields about 2 cups of mashed pumpkin."
Hope that helps. If you are interested, I have a really good pumpkin bar recipe with cream cheese frosting that I usually make quite a bit (works great for bake sales because it makes a lot). Also, somewhere around here a recipe for a pumpkin toffee (I think) trifle dessert. I would really have to search for that one, but you are welcome to both if you would like. Good luck - I've never tried to actually cook a pumpkin before, I just always used the canned stuff so let us know if it works for you! C. :)