I wouldn't think a small size amp would be better for a uke. It needs to be able to generate tones in the sound range you need, but that doesn't mean smaller is better becaue you have a smaller instrument. The bigger issue for me is whether it has enough power to create clean tones with decent volume. That's much harder to do with small amps that have lower power settings. There are some amps that work better with acoustic instruments though, vs solidbody instruments. They handle feedback better I think and there are probably other differences too. So presumeably the kala is an acoustic mini amp.
I have a Vox DA5 which I love - it can run on AC or regular batteries and has a lot of very good on-board FX like reverb, distortion, delay, and chorus. Fits great in a small rolly dj bag. Love it but whilte it's certainly portable, it ain;t a belt buckle amp either... I like it better than the Roland Cube, though it's possible the Cubes have improved in tonal quality and presence since I was last shopping....