If the warp engines, window rows and bridge dome are any indication, the Jenolan
was about the size of Kirk's original two ships. And the huge black opening aft appears to be a cavernous shuttlebay or cargo hold - indeed, it's difficult to locate any impulse engine (that is, any rocket nozzle shape) on that ship at all, unless it's immediately adjacent to the blue dome atop the hull.
Still, it is plausible that a smaller ship would have an advantage over a larger one in this respect, even when those fancy mass-nullifying fields come to play. The [i]Jenolan[i] looks like she could dip into an atmosphere and perhaps land and take off, probably calling for plenty of off-axis thrust that might also aid in spatial maneuvering. And she'd probably have different impulse characteristics empty and fully laden, while the E-D would have lesser differences in that respect. I wonder what else was being transported to Norpin besides Scotty's personal bulk?