Mitochondria don't really have a fixed shape, and so it would be imprecise to call them "circular." Furthermore, circles are two-dimensional shapes, like you would draw on a piece of paper, and mitochondria are three-dimensional objects; the appropriate term would be "spherical" if we want to describe them as being a circular three-dimensional object.
In most textbooks, mitochondria are depicted as being something like a sausage, oval or elongated circle, and usually in cross section to reveal the numerous and intricate foldings of their membrane, where ATP synthesis takes place. Mitochondria may even be tube-like. The point is that mitochondria are dynamic, meaning they respond to stimuli, and their shape seems to depend strongly upon the type of cell they're in, the immediate environment around them, the age and condition of the organism, and amount of energy demanded of them.
If mitochondria, at any location or time in their life, are spherical, this may simply be a matter of efficiency. The sphere is the most compact and surface-to-volume-ratio-efficient shape that the mitochondrion can take while retaining its function and identity, and the balance between volume, surface area and ATP production is a careful balance that may favor multiple spherical mitochondria in one circumstance, or fewer, elongated mitochondria in another.
Other possibilities not related to the functionality of a spherical shape include the chance that we're seeing the mitochondrion during division or fusion, or we may be seeing a cross-section of a more elongated mitochondrion that appears circular from that particular plane.
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