Surface area and volume
For a cube with side length s, surface area is 6s² and volume is s³, so SA:V is 6/s. For a sphere, area scales with radius squared while volume scales with radius cubed. Increasing size therefore reduces the amount of exchange surface per unit volume. Cell shape can offset some limitations: flattened cells and membrane projections provide more surface relative to volume. Internal membranes increase reaction surface but do not automatically increase the outer boundary available for exchange. Real transport rates also depend on gradients, permeability, and transport proteins; SA:V alone is not the entire story.
A cube with side 2 μm has area 24 μm² and volume 8 μm³, giving 3 μm⁻¹. A cube with side 4 μm has area 96 μm² and volume 64 μm³, giving 1.5 μm⁻¹. The larger cube has more total area but less area per unit volume.
Do not confuse larger total surface area with larger SA:V. Always divide area by volume and keep the units sensible.