Here's the thing. Pumpkins themselves have very little flavor. They're a very plain gourd that lend little to a beer other than bland starch. The smaller pie sized cinderella variety sugar pumpkins or even the blue jarrahdale blue pumpkins are far sweeter and better suited to brewing than your average big bland watery jack o lanterns.
The flavor in most pumpkin beers comes from pumpkin pie spices. In fact, nearly all commercial brew houses use just the spices to flavor their pumpkin beers and not actual pumpkin itself. A larger proportion of sweetness in these beers comes from larger additions of crystal malts, usually crystal 40 and crystal 60 (very caramel flavored crystal malt). Honey malt is also a nice addition to pumpkin beer recipes.