lewm, I agree that it certainly doesn't look like a low mass arm. I really doubt that it is. My Denon DL-103 played pretty well on it, actually it was the UA-5045 tonearm that I paired it with but they look almost identical from the top of the mounting base on up. At least the parts of the arm that move are pretty much the same.
I think it works well on higher compliance cartridges because it's designed and built very well. Here is a description from VE.
"The high-quality JVC UA-7045 gimbal support tonearm features high sensitivity and long lasting ruggedness.
It permits low-mass, high-compliance cartridges to accurately track even the most complex groove undulations.
The ultra-effective support system converges the effects of forces on both the horizontal and vertical planes on one point equidistant from both the horizontal and vertical bearings.
Thus in principle the new gimbal support support of the UA-7045 works like a one-point support and is therefore very sensitive.
However, its performance is far better than conventional gimbal or one-point types since it uses adjustment free bearing units.
Specifications
Type: statically balanced arm with new gimbal support for tracing hold system
Length: 350mm
Effective length: 245mm
Overhang: 15mm
Tracking force: 0-3g (0.1g steps)
Cartridge weight: 12-32g (including headshell weight)
Arm elevation range: 40-60mm
Weight: 610g
Accessories: oil damped arm lifter, low capacitance signal cable"
This description comes directly from JVC, not something that VE wrote up. Pull up the manuals for this arm and read the JVC literature. Note, don't look under Victor, it's listed under JVC.
BillWojo