As discussed above, a heart attack is a process where an artery
feeding the heart muscle with oxygenated blood becomes blocked or is obstructed, to the point where "infarction" occurs. Infarction is deprivation of oxygen causing cell death.
Differentiating a "massive" heart attack verses a regular heart attack does not have any specific medical criteria. Usually we talk about a massive heart attack
in terms of the end outcome..."he had a massive heart attack and died" or "he had a massive heart attack, he's in intensive care and its touch and go." Now, why would one type of heart attack be minor and another be massive?
Ateries are like trees. Large ones feed smaller branches, etc. So a massive heart attack has to do how far upstream the artery is blocked, what the blocked artery feeds, the extent of the blockage, and amount of time it took before reprofusion was accomplished. There are very large arteries that are the main arteries feeding others - Left Main and Left Anterior Descending, for instance. The left side of the heart is the largest and the one responsible for pumping blood through the body. So any artery feeding the left side heart function is critical.
If its one of the two arteries mentioned above and its a complete blockage then death or mortal injury
is very likely unless you are surrounded by medical people when it happens.
On the other hand if a small artery is 85% blocked downstream and feeds the right side of the heart; then by blood test
or EKG the person has had a heart attack, but might have walked around with it for days.
As you can see these are very different events - both heart attacks - one massive and one minor.