Two InAs/In0.15Ga0.85As quantum dots-in-a-well (DWELL) samples have been grown by solid source molecular beam epitaxy (MBE). The increased size of InAs dots and more homogeneous dot-size distribution have been found in one InAs DWELL sample with growth optimized by changing both the growth temperature and the thickness in In0.15Ga0.85As well layers. The improved optical properties of this sample have also been confirmed by photoluminescence (PL) and piezomodulated reflectance (PzR) spectra. The numerical calculations based on effective mass approximation indicate that the increase of dot size and the improved optical properties are dominantly due to the strain driven alloy decomposition of In0.15Ga0.85As well layers.