Traditional ADPCM is not terrible, it's just "meh". Compare:
- Original, 4039kb: https://phoboslab.org/files/qoa-samples/adpcm_comp/orig.wav
- MS ADPCM, 1022kb: https://phoboslab.org/files/qoa-samples/adpcm_comp/ms_adpcm....
- QOA, 812kb: https://phoboslab.org/files/qoa-samples/adpcm_comp/qoa.wav
IMA ADPCM is in the same ballpark as MS ADPCM. ADX (not listed) comes close to QOA, but at 1136kb is also larger than those traditional ADPCM flavors.