Enlightened by the approaches taken in other answers and encouraged by the large number of views, I suggest this analysis as an answer to my own question.
There is agreement on the web that dressed weight is about 50% (e,g, sites below). There is less agreement on the weight of a lamb of a year of age or under.
This site gives the weight of lambs at 12 months as 20 kg or 44 lbs.
This site says the avg weight is 160 lbs at 8 months.
This site gives weights of 33 to 55 kg (73 to 120 lbs) for ages of lamb ranging from 4 to 11 months, with the 33 kg for unweaned 4 months of age.
This site says goats gain 1/3 to 1/2 lbs per day and reach 30 to 40 lbs in 66 to 69 days.
It says lambs gain .4 to 1 lb per day.
So 3 of these sites that give lamb weight at a year report: 44, 160, and 120 lbs. The low average seems to be an outlier but I will not throw it out. The average of these 3 estimates is a little over 100 pounds, or dressed weight of 50 lbs. One site said fats and organs are 3% of weight, so I am ignoring them.
Dressed weight includes bone. This site says some 70-75% of a lamb carcass is meat.
But raw meat weight is not cooked meat weight. This site says that roasted lamb yields 74%.
So, a 100 lb lamb yields 50 pounds of dressed meat, or about 36 lbs of meat to roast, or about 27 pounds after roasting. Assuming a 1/8 pound serving of meat, that comes to 214 people per korban pesach, on average.
A small lamb of 30 lbs would yield a 1/8 lb serving for 64 people.
At lamb at birth weighs about 7 lbs (per one of the above sites). Assuming the above factors apply, that would be 15 people.
What if the participants were really satiated before eating the koran pesach (perhaps due to eating the korban chagiga), and ate only a k'zayis of meat. To find out the impact, we need to estimate the weight of a k'zayis of meat. I tried to do that as follows.
If a k'zayis is a cube 1.2" on edge, or 1.73 cubic inches, and if a cubic foot of water weights 62 lbs, and if meat is mostly water, then one kezayis of meat would weigh 1 oz (if I did the math correctly). If people only ate one k'zayis of meat, the above numbers would have to be multiplied by 2 (since there are 16 oz in a pound or 2 oz in 1/8 lb,)
Perhaps lambs are bigger now than they were in the past. Even so, it seems the seders were large back then.