Solar Team Eindhoven
I’ve been hearing some curiosity about the sizes of solar car teams, and so I checked out the online team lists for Punch, Bochum, Twente, Eindhoven, Nuon, Lodz, Michigan, MIT, PrISUm, and Sunswift. The histogram below summarises what I found. The superb Bochum team is the largest, with 77 members. Champions Nuon have the smallest team, with 16. Apparently it’s not just size that is important.
See also my list of WSC solar car teams.
Scientific Gems 
Right, what’s really important is the time commitment that the team members are able to make. The dutch teams have the luxury of taking a full year or more off of school, and others like Michigan are able to recruit a race crew that can financially afford to forego a paid internship over the summer and work full time on solar car.
You’re right: time commitment is one of the key factors. And working on a solar car is essentially an unpaid internship — unpaid, but a great thing to have on a CV (some students are also able to incorporate it into university work to a greater or lesser extent, e.g. as an Honours or Masters project). It’s not easy. Having sufficient travel funds helps, and that in turn depends on how good the fund-raising/PR/media sub-team is.