I happened to write a paper on just this issue (shameless plug)! You're right: With a sphere, the metabolic consumption grows like R^3 but nutrient capture go

I happened to write a paper on just this issue (shameless plug)! You're right: With a sphere, the metabolic consumption grows like R^3 but nutrient capture goes like the surface area, which scales like R^2. The result is a max radius R where nutrient in can support the consumption. However, for long thin object, both the volume and area grow like L, the length. The result is that -- rather than plateauing to a finite limit -- a long thin bacterium can actually maintain exponential growth indefinitely! and that is what is actually seen in the lab. https://arxiv.org/pdf/1312.0674.pdf https://arxiv.org/pdf/1312.0674.pdf