Two luminous debris discs around 100 Myr old have been searched for 1 cm dust emission, to a depth three times greater than in any such previous study. Very low limits were set towards both HD 377 and HD 104860 (noise levels of 12–14 Jy), extending the spectral range from 70 m to 3 mm where cool dust has previously been sought. The almost-identical fluxes of the two systems were merged into an average spectral energy distribution, which was then fitted using a distribution of grain sizes. The canonical infinite collisional cascade, with numbers of particles n(D) ∝ D−3.5 for diameter D, overpredicts the 1-cm flux, which should have been detected at over 3σ for the merged system. Preferred solutions have truncated grain populations with largest particles around 1.5–4 mm in diameter, and slightly flatter distributions, up to n(D) ∝ D−3.1. The lack of cm-sized and larger particles is reminiscent of the similar deficit inferred around comets from spacecraft encounters. Such departures from simple power-law distributions of particles have been predicted in recent models where some size regimes are more readily broken up than others. A deficit of cm-scale particles can explain the fits we obtain, and reduces the total masses inferred for the comet belts of these stars.