I just bought a new Corsini roundback from the Double Bass Workshop in Madison. I'd be happy to take some measurements if it would help. As mentioned above, the 3/4 designation covers a lot of ground. How wide is the bass at the upper bouts? I get just a tad under 20" on the Corsini. The two models also have different shapes of their "shoulders," as represented by the pictures at the importer's website.
http://www.jrmusicsupply.com/basses2.htm
Feel free to ask about anything, though I am far from an virtuoso or a qualified teacher. The Corsini is probably more bass than I deserve, but it really stood out as having a big clear tone, especially for not being a huge bass.
As for your height, I'm 5'10" and find the Corsini pretty easy to get around, but I've seen shorter players handle a 3/4 just fine. And the Carcassi isn't a giant. I think it's worth analyzing closely what's going on between you and the bass, like Meddle suggests.