Here's a link to an article by one Alexis Madrigal that disses the modern turkey, modern corn, and modern potatoes as oversized, oversweet, and genetically "hacked". It has a chart, supposedly illustrating the growth of the average turkey. According to the chart, in the 1920's the average turkey weighed about 13 pounds, today's turkey weighs about 29 pounds. There's no source cited for the chart (though mousing over shows the chart title to be "new_sweet_chart"??
A brief session of Googling doesn't turn up any facts, so I'm not sure what they are, except Mr. Madrigal's chart and statements are misleading, at the least, and most likely wrong. Let's start with the concept: "average turkey". My wife and I have been having turkey since 1980 or so, each time we buy the same size bird: 10-12 pounds. Given the American household has shrunk in size over the years, I think it's safe to guess that "average turkey weight" does not mean: the average weight of turkeys sold at retail in the U.S.
So, could "average turkey weight" mean the genetic potential--what would a turkey weigh if it grew to its maximum weight? Well, probably not. From the heritage turkey page at Rodale comes this paragraph:
Heritage birds command a premium (consider a store-bought turkey at 39 cents per pound) because of their genetic value and added labor costs. They are, on average, much smaller birds (10 lbs for hens, 12 lbs for toms) that take twice as long to mature as the Large Whites. Still, Frank Reese, an experienced heritage turkey farmer (Good Shepherd Ranch in Linsborg, Kansas, www.reeseturkeys.com), estimates that if done properly, growers can make a nice profit of $60 to $80 per bird. Thanks to careful selection and breeding, his heritage birds average 18 - 33 pounds. (Reese and other heroes in conserving heritage turkeys are recognized by the ALBC at www.albc-usa.org/alerts/Oct13_03.htm)So heritage birds can reach 33 pounds. (The Diestel Family Turkey Ranch advertises such birds.)
For a turkey grower I'd guess the two metrics most important are weight gained per pound of food and age to marketable size. Madrigal does give a sentence to this, crediting modern turkeys with being very efficient at converting grain to meat and being twice as fast to market. But it's a lot more sexy to say: "Science Supersized Your Turkey Dinner" than to say: "Science Made Your Thanksgiving Dinner Both Energy-Efficient and Bland." (Less grain for the same meat is more energy efficient.) By focusing on size rather than efficiency, Mr. Madrigal skews his piece.