The small one, made for those who wish to not fiddle before listening to their favourite piece of music.
As per Sony's then current numbering scheme, the TA-1055, lower-numbered, is the bigger one, the TA-1066 is the small(er) one.
The main differences are a supplementary input, an IC preamp stage and an effective external heatsink on the back.
Both are DC design, both had a stock wood-enclosure and both are very good-looking, the TA-1055 especially - perhaps more than the big separates Sony had then available... can't have it all !
Made to be amplify the ST-5055 and ST-5066 tuners - equally unpretentious, equally good-looking :)
A miniseries of two which, in Japan, was augmented by two : TA-1070 (june '74) and TA-1090 (july '74).
A miniseries of two which, in Japan, was augmented by two : ST-5070 and ST-5090.
Not many Watts per channel but these were 1970s Watts and they still are !