We investigate the influence of the addition of polymer chains on the effective interaction between star polymers, as a model for the depletion potential in ultrasoft mixtures. The effects of size ratio and chain polymer concentration on the chain-modified star-star interactions at good (athermal) solvent conditions are investigated. For both hard sphere mixtures and colloid-nonadsorbing polymer mixtures the range of the depletion interaction increases with the size ratio. For the systems at hand, the range of the depletion potential is insensitive to the size of the depletant polymer. The physical origin of this and the associated effects, as well as a mapping of the mixtures onto a one-component system, are discusssed.