The critical thickness for two-dimensional layer growth of Si1−xGex on Si and InxGa1−xAs on GaAs is about 1–3 monolayers (ML) for x=1, beyond which islanding begins. Under certain growth conditions, this thickness t would increase as some power of 1/x. The reason for this is not clear; Snyder et al. argue that, under equilibrium conditions, this critical Stranski–Krastanov (SK) thickness tc is independent of x and should remain at 1–3 ML, but that, under nonequilibrium growth conditions, t∼x−4. However, Osten et al. showed, experimentally, that even under equilibrium conditions there is an increase of SK thickness with 1/x. We carry out calculations of energetics of large three-dimensional (3D) islands on substrates with varying thicknesses t of the epilayer and different coverages θ. We show that at low θ or when islands are small (or both) then the SK thickness is small ∼1–3 ML, in agreement with the results of Snyder et al. At increasing coverages, when interisland separation l decreases to the point where l∼s (island size), we observe ΔE to decrease for the lower thicknesses t=3,4,5,…8…; until thicknesses t>3 become more favorable. There is considerable tension going deep into the substrate directly below islands. The larger an island becomes, the more favorable it is for a thicker layer beneath it to be of the same material as the island. It is known that the critical size sc at which 3D islands first become favorable varies as x−2. We argue from this that, at equilibrium, the average 3D island size increases with x−p, p some exponent, and at high enough coverages, when interisland separation is small, SK thickness tc increases. The experimental results of Osten et al. are consistent with exponents 2⩽p⩽4.