I don't think it's a major issue of the tub not being big enough or anthing lke that, it's more about having to adapt the one car they took to the test from Vettel's settings to accomodate Webber's height. I also don't think that the conversion took all the time he lost, I think the pit crew found other issues with a master cylinder failure they had to sort out. Below there's a piece on ESPNf1 from last Wedneday explaining some of the changeover problem and a snippet from f1paddock.biz talking about the master cylinder.
Mark Webber had to wait until the final hour of the second day of testing before finally getting out onto the track. "I didn't get so long in the car today," he shrugged, "so we'll start the real work tomorrow. There was a bit of a problem with the installation, something with the pedals, so it took a bit longer than we expected.
"It wasn't the smoothest day, but we're back tomorrow. It's great that the guys have got the car here for the first test and I'm looking forward to driving it more."
Sebastian Vettel, who clocked the second fastest time behind Fernando Alonso, enjoyed another good morning in the RB7. "Sebastian pretty much ran to schedule this morning; there were a few minor delays, but nothing significant," Ian Morgan, the team's head of engineering, said. "Unfortunately we lost some time changing drivers this afternoon due to some driver installation issues. Once Mark's testing session got underway, he was able to complete two runs and begin acclimatising to the new car."
© ESPN EMEA Ltd
Mark Webber declared himself satisfied with the initial performance of the Red Bull RB7 following his first representative run in the new car on Thursday.
After a master cylinder failure severely curtailed his running time on Wednesday, the final day went much more smoothly as Webber completed 105 laps and finished fourth fastest.
f1paddock.biz, Feb 3