Mass-size relationship of the ATLASGAL Top100. The lower right gray shaded area highlights the region where it is unlikely that high-mass stars are formed according to Kauffmann et al. (2010) and the black dashed line indicates a surface density of 0.05 g cm^-2 determined as the lower limit for efficient massive-star formation by Urquhart et al. (2014c). The slopes of the linear log-log fits of all classes agree with each other within the margin of error of the fitted power laws.