The beaming angle (ΔΩ) as function of the brightness temperatures (T_b) for all the unsaturated CH₃OH masers detected in NGC 7538 (Surcis et al. 2011a), W51, W48, IRAS 18556+0138, and W3(OH). The arrows indicate that the masers are unresolved, i.e. the maser angular size Σ is less than the beam-size of the observations and consequently ΔΩ is smaller and T_b is larger. The blue line is the best linear fit of the resolved masers and the dashed black line is the theoretical proportionality ΔΩ ∝ (T_b)^-1 (Elitzur 1992).