Despite the wider application of efficiency analysis in Indian agriculture, little has been done on the investigation of intra and interecological variations, size differences and their interactions. However, ecological issues have paramount implications for the low-input sustainable agricultural production. Furthermore, due to the various causes of efficiency, the age-old size-based debate on efficiency differences is not yet resolved. The present study examines the level of technical efficiency across ecological zones and farm size groups in paddy farms of the southern Indian state of Tamil Nadu. The study showed that 90% of the variation in output among paddy (IR-20) farms in the state is due to differences in technical efficiency. Land, animal power and fertilisers have significant influence on the level of paddy production. Varying from 0.59 to 0.97, the mean technical efficiency was found to be 0.83. The use of F-test in two-way analysis of variance (ANOVA) and censored regression (Tobit model), with dummies for ecological zones, farm size groups and their interactions, has shown that, at mean level, the level of technical efficiency among paddy farms of the state differs significantly across agro-ecological zones and size groups as well. The study further indicated that small-sized paddy farms in zone II and medium-sized paddy farms in zone HI are represented by ecologically size-biased production techniques: thus achieving higher technical efficiency.