Since bacteria have few ways to transport nutrients across their cell membranes, most are small.  They rely on diffusion to move materials like food and wastes in and out of their cells.  This is limited by the surface area of the membrane, which is the space it would occupy if stretched out flat.

Imagine a bacterium as a paper bag.  Each paper bag holds a certain amount (its volume), while made up of only a small amount of paper (its surface area).  A large bag may hold the same amount as several small bags, but laid flat the large bag has much less paper.  The same is true of bacteria.  Small cells require little food, so they can get enough nutrients across their small membranes.  As a cells grows, it requires more food.  The cell’s surface area can’t grow as quickly as its volume, limiting the amount of food absorbed, and therefore the size of the cell.  This balance keeps most bacteria very small.

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