'Miller High Life Guy' -- Died From Fatal Blood Clot

Windell D. Middlebrooks -- the Miller High Life delivery guy -- died from a pulmonary embolism ... TMZ has learned, but we're told weight was definitely a contributing factor.

Our sources say the autopsy is clear ... Middlebrooks' 400 lbs made him a target for health problems. What's interesting ... his family tells us before his death Middlebrooks was relatively healthy with no significant medical issues.

TMZ broke the story ... 36-year-old Middlebrooks died Monday in his San Fernando Valley home.

We're told Middlebrooks' family is trying to get his body for a burial.