A hypothesis that postchallenge hyperglycemia in subjects with low body weight (BW) may be in part due to small glucose volume (Gv) was tested. We studied 11,411 non-diabetic subjects with a mean BW of 63.3 kg. 5,282 of them were followed for a mean of 5.3 years. In another group of 1,537 non-diabetic subjects, insulin sensitivity, secretion, and a product of the two (index of whole body insulin action) were determined. Corrected 2 hour-plasma glucose (2hPG_corr) during a 75 g oral glucose tolerance test in subjects with BW ≤ 59 kg was calculated as, 2hPGcorr=PG__2h ·ECW/[16.1 (males) or 15.3 (females)] + fasting PG (FPG) where PG_2h and ECW denoted PG increment in 2 hours and extracellular water (surrogate of glucose volume (Gv)), respectively. 16.1 and 15.3 were ECW of males and females, respectively, with BW = 59 kg. Multivariate analyses for BW with adjustment for age, sex and percentage body fat were undertaken. BW was, across its entire range, positively correlated with FPG (P < 0.01). Whereas BW was correlated with 2hPG and PG_2h in a skewed J-shape, with inflections at around 60 kg (P for non-linearity < 0.01 for each). Nonetheless, in those with BW ≤ 59 kg, insulin sensitivity, secretion and action were unattenuated and incident diabetes was less compared to heavier counterparts. BW was linearly correlated with 2hPG_corr, i.e., the J-shape correlation was mitigated by the correction. In conclusion, postchallenge hyperglycemia in low BW subjects is in part due to small GV rather than impaired glucose metabolism.