Thanks for accepting what I've found as an answer. I had another look
at the information available about the data and there is no indication
whether the data is split out by size of practice. (However, there's
also no indication that it isn't either!) It is unfortunate that they
wouldn't be more specific with me when I emailed them.
Expense survey: What to spend, what to cut
This article has two charts that would be of interest to you:
What 10 Specialties Spend on Overhead
What Other Specialities Spend On...
This data was gathered from the industry standard, "Statistics:
Medical and Dental Income and Expense Averages, 2004 Report Based on
2003 Data", published by the National Association of Healthcare
Consultants, The Society of Medical-Dental Management Consultants, and
the Academy of Dental CPA's.
To obtain the full report costs $795 for non-members - but a single
specialty report can be obtained for $249 for non-members. More
information can be found here:
National Association of Healthcare Statistics - Statistics Program
Quote: "The report contains information from over 42 states, 6,500
doctors and 3,200 practices. It covers 56 Dental and Medical
Specialties Including Family Practice, Internal Medicine, Obstetrics &
Gynecology, Psychiatry, General Dentistry, Orthodontics, Veterinary
Medicine, and more."