In at least some parts of Germany they're more likely to encounter narrow streets where the typical US brown UPS truck might have real difficulty navigating.

I also suspect that the routes are shorter than you might think, because there are a lot of stops and they are not moving all that fast for most of the trip. I suspect most UPS trucks probably average well under 25 miles per hour for most of their travel which would put an 8 hour day around 200 miles. In denser areas like cities it could be significantly less than that. If a truck makes a 1-minute stop to toss a package on your porch then drives half a mile in another minute and repeats, it's still only covering 15 miles and 30 packages an hour.

I wouldn't be surprised if UPS trucks within cities tended to be below 50 miles/day most of the time, and since they control driving routes very tightly they can easily put these trucks only on appropriate routes.