Chief,

Short-priced horses do have a SLIGHTLY better record in shorter races although there are not really that many races over the long distances (2100M+) and they tend to have larger fields so as you say it is probably not comparing apples with apples. For the record I did some analysis on Short races (1200M or less), Medium (>1200 and <2100) and Long (2100M+). These figures are based on one years data:

Number of Races: S=1400, M=2012, L=274
Winners $1.00-$2.00 S=8%, M=6%, L=6%
Winners $2.10-$5.00 S=42%, M=41%, L=35%
Winners $5.10-$10.00 S=28%, M=28%, L=34%
Winners $10.10-$20.00 S=14%, M=16%, L=14%
Winners Over $20 S=8%, M=8%, L=11%

However as you noted there are more short priced runners in the shorter races. If we look at the success rate of those runners (ie. what percentage of runners in a given price range win) we get the following:

$1.00-$2.00 runners who win: S=47%, M=50%, L=40%
$2.10-$5.00 runners who win: S=25%, M=25%, L=24%

NOTE: The 40% figure for Odds-on runners in Long races is based on only 110 runners so you need to treat it with caution - not a big enough sample size to guarantee accuracy.