In this case, muck-raking is the discovery of events and incidents while yellow journalism is the publishing of the results.
My old journalism teacher would roll over in his grave if he read that statement.
Yellow journalism is the intentional incorrect sensationalism of a news story to induce certain reactions to the reader. If a muck-raking story is true then it is not guilty of yellow journalism regardless of the headline type size.
The initial investigations of the sinking of the Maine all concluded it had been attacked with a mine. The theory that firedamp (coal gas) had ignited in a bunker came much later. In the meantime Hearst printed that their reporters had concluded that a mine was used to sink the vessel.
If you read the total history of the USS Maine incident you will find that Hearst began the drum-beat against the Spanish before any positive cause was revealed. It was assumed, early on, that either the Maine had contacted a mine (unlikely since it was anchored) or a saboteur had planted a bomb aboard. Hearst began clamoring for retaliation against Spain (Cuba was a Spanish possession at the time) while the investigation was still underway.
There are many other examples of Hearst yellow journalism, a good many of them directed at Hollywood people even though William R. Hearst had actress Marion Davies as his live-in girlfriend for many years. He was truly a despicable human being.