It is well known that new businesses are typically much smaller than their established industry competitors, and that this size gap closes slowly. We show that even in commodity-like product markets, these patterns do not reflect productivity gaps, but rather show differences in demand-side fundamentals. We document and explore patterns in plants’ idiosyncratic demand levels by estimating a dynamic model of plant expansion in the presence of a demand accumulation process (e.g. building a customer base). We find that active accumulation driven by plants’ past production decisions quantitatively dominates passive demand accumulation, and that within-firm spillovers affect demand levels but not growth.