Yup, size is a factor in multi-vehicle accidents but it's not the only factor. Construction and air bags make a big difference too. In single-vehicle accidents size is irrelevant and crumple zones and airbags are everything. Car safety has improved significantly over the years. Here's an interesting head-on crash of an older Volvo against a newer Renault.
http://www.youtube.com/watch?v=k3ygYUYia9I

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL