Aww, Ian!
And I thought this only happened to me on my MagLite! It is hard work getting the out again once wedged firmly in place. I finally resorted to super-gluing a coiled paperclip onto the negative pole of the battery, waiting for it to cure, then extracted it that way with some additional effort and Vise-Grips/Mole-Grips.
Yes, the diameter of AA-cells varies, and more in practice than AAA-cells do. Sometimes, the shrink-wrapped label is enough to do it.
<nods> Yep, the onus is on the device-maker to follow battery size standards, and it appears CatEye didn't in this case, though molding flash on the light can cause the same effect, especially midway in the bore. I used a small-diameter expanding reamer/hone t solve the problem on several of my devices; it doesn't take much, but is a lot of bother and one only realizes there's a problem after the fact.
Best,
Dan. (...who reminds himself: While it is nice to have a hobby, there's better ones than battery extraction)