I would say (without much evidence I must admit) that a profit margin of 10% in this industry is rare, AECOM as an example (not a good one) has a profit margin of 0.68%. I will sympathies with the point about conglomeration of the economy being distasteful and potentially dangerous but that is probably a separate discussion in itself. With that said, I think the point CWB1 makes is probably closer to the truth. Yes AECOM could be seen as being bloated with too much bureaucracy and policies, and paper pushing departments but that is what makes a company of 87,000 people function well. That is also where you gain your economy of scale since each 500 person subsidiary does not need a receptionist, HR manager, accountant, lawyer, etc. they can just have engineers, architects and PMs. The size of the company also is what makes it possible to get jobs, the reality is a municipality, federal government organization or fortune 500 company does not want to trust the design and construction of their new building/road/sewers to a 200 person firm that has maybe done one project of that scale when they can have AECOM do it. AECOM can also bring everything under one roof which is a significant savings (and helps the overall bottom line), they can handle the engineering, architectural, project/construction management, design of the utilities, probably even landscaping and facility management once its built. They can also finance these projects and bond for the large projects where small firms can not.
I agree there is a discrepancy in the small firm vs. large firm valuations but its similar to baseball player salaries. You can have a 5-tool baseball player who makes $20M/year and on the same team have a 2-tool player who has a higher batting average than the 5-tool player but makes $3M/year. Yes he can do the same thing in the batting department and by all accounts should be rewarded for that commensurate with the 5-tool player but the full package is worth more than then individual parts in some regard.