I think that the amount of material should not really be a bottleneck for development. Using the butterfly logic, There is still a need for the capacity to turn specialized materials into base resources, so a good way to balance this would be a weighted cost system that allowed exponential-decay costs (another idea is to make the accumulation rate increase), that allowed specialization, and perhaps an increasing cost to develop body mass, which would act as storage space for fitting in more materials. This would mean that a bot with plenty of muscle would have trouble developing fat (but still has the ability to do so), but could still convert that muscle into fat with vastly reduced cost. In this case, the bottleneck for development should either be a.)making more free space or b.)the conversion of resources (which could have other benefits, eg. fat can be digested faster than chlorophyll, so can act as a space/energy holder).
I have a feeling I failed to address about a dozen arguments. Feel free to remind me of anything I may have ignored.