Despite their massive size, the throat of a Blue Whale is physically unable to swallow any object larger than what common piece of fruit?

A grapefruit

The grapefruit comparison is a rough analogy, not a scientifically established maximum: published anatomy research describes the commonly cited blue-whale measurement as anecdotal. Measurements from fin whales, close relatives that feed similarly, nevertheless confirm that rorquals have remarkably narrow esophagi relative to their enormous bodies.