I haven't run web2py in a huge scalability setting before, but I know there are a couple in the forum that have. I know that last weekend, the web2py.com site had about 262,000 hits in one day, and I think that is running on a small VPS.net instance (if I remember correctly). The web2py.com site is actually running web2py, so that should say something. The result of most of the scalability questions is that the database is usually the thing that slows everything down (something that .NET developers have to deal with as well).
Web2py really wants you to use MVC. I've never used .NET MVC3, but I can tell you that web2py makes a clear distinction of the MVC pattern by using folder names that coincide with each part (models, views, controllers). The main reason for big shops to use .NET is mostly the corporate backing of MS. They offer these shops some incentives when they start out to get them "hooked", then they charge through the nose after a while. I haven't used .NET for web development in quite a few years, so I can't comment on its current state. What I can tell you is that I could not have accomplished some of the things I have done with .NET, Java, Django, etc. The only language/framework combination that gave me the power and flexibility I needed was Python as web2py. As for the size of projects, there are quite a few (including myself) that are developing ERP applications (if you don't know what those are, think of the most complicated business application ever, ever that can basically run an entire company by itself). I have also developed an appliance using web2py that assists in the deployment and provisioning of servers. I think imagination is the only limitation here :)
