Let's see if we can find any empirical evidence of an association between body weight and the amount of levodopa required.

Here's a scatter plot of PDMeasure data showing, for each gender, the levodopa equivalent daily dose and the number of years since diagnosis.

genderDosage.png

Statistics for men are:
n=24
mean years since diagnosis = 4.97
mean LEDD = 413mg
r=-0.045 - this could be caused by an outlier
assumed mean weight=190lb - based on Wikipedia[1] and UK, US bias in PDMeasure

Statistics for women are:
n=14
mean years since diagnosis = 3.98
mean LEDD = 288mg
r=0.671
assumed mean weight=158lb - based on Wikipedia[1] and UK, US bias in PDMeasure

Note that in this sample women are on average more recently diagnosed (by 1 year) and have a lower LEDD (by 125mg). If we scale up the women's figure to take account of the different average body weights of the genders, 288mg goes to 288*190/158=346mg. There's insufficient data to get statistical significance, but, in my opinion, the data is more supportive of the notion that the titration process gets people regardless of weight to the right place for them, rather than lighter people are being generally over medicated. But that certainly does not mean that no one is over medicated.

Laura raises two points:

Can LEDD values be used to estimate progression?
The men's figure suggests not, but there's a particularly prominent outlier that may be distorting things. However, the women's figure shows a moderate correlation between LEDD and years since diagnosis which is, in turn, given the progressive nature of PD, probably linked to progression. Correlation is not transitive. But it seems reasonable to assume LEDD is associated weakly with progression. Further work is needed to pin down the direction of causality.

The importance of biomarkers.
I agree with that, but feel that not enough is being made of the objective symptom measurement tools that we already have. If the will had been there, we could have mass data collection by now.

References

[1] http://en.wikipedia.org/wiki/Body_weight

John