It's a buyers market subject to supply and demand.
Buyers want large chainsaws much more than they want smaller chainsaws.
There are many more smaller chainsaws available than large ones.
Many smaller chainsaws for sale + a small group of potential buyers = low prices.
Very few large chainsaws for sale + a group (doesn't need to be large) of determined buyers = high prices.
Consider many of the rebuild, and restoration, threads here on ArboristSite. With the exception of some threads from the saw modding guys, and perhaps the Stihl MS 200T, the majority of those threads feature larger saws. Some do rebuild smaller saws, but usually have ceiling on the cost. Not many people are to do a full restoration on a smaller chainsaw. The dollar value is a factor, but perhaps the biggest is the "Wow!' factor, which clearly favors larger saws.
Think about it--Which is more impressive (the Wow! factor), a restored Poulan 8500, or a restored XXV? Both are good saws, and in reality for many folks the XXV would be more practical & useful, but when it come to time and effort (and recouping some money should the saw ever be sold), the 8500 wins out.