McClellan also had a close relationship with Allan Pinkerton -- of "the Pinkertons" fame, yes, that Pinkerton -- who was the head of his intelligence apparatus...
https://en.wikipedia.org/wiki/Allan_Pinkerton#American_Civil_War
Pinkerton was notorious for supplying McClellan with faulty intelligence that overestimated Confederate strength and numbers. Pinkerton was either being double-crossed by his sources in the South and/or falling for some of the oldest tricks in the book. Some examples would include the Confederates lighting an unusually high number of campfires at night relative to their actual numbers (which Pinkerton and the Union would count assuming far too many men to a fire) or painting logs black and putting them on mounts and towing them around. They look exactly like real cannons from a distance.
https://muse.jhu.edu/article/420411
There is a camp of Civil War historians who paint McClellan as the victim here. That he was a brilliant administrator and organizer who built the Union Army into a fighting machine from virtual scratch, and he was overly cautious as a commander in the field not for any failings of nerve, intelligence, or daring, but rather because he was dumb enough to listen to (and continue to listen to) faulty information. Listening to that information demanded care and caution, so McClellan eventually takes the fall for that, even if his own doings and decisions were not the kernel of the failings there. They claim, even for his faults, he deserves credit for creating the weapon that more aggressive commanders like Grant, Sherman, Sheridan, and Thomas eventually wielded in the victory over the South.
Once Grant showed up, he stopped listening to such suspect warnings. He dismissed them as preposterous given the difference in population and industrial might between the Union and the Confederacy. His own experience fighting in the West, where he always had superior numbers, equipment, and logistics to his opponents, probably strongly informed that view when he moved to Virginia for his showdown with Lee.
Pinkerton constantly putting a bridle on McClellan is one of the greatest intelligence failings in American history. This is true if you believe that McClellan was being prudent in his failings OR if you think this just gave the guy polishing his car for the third time today yet another excuse not to take it out and drive it. Either way, telling a commander he faced 50% to 100% more than he actually did much of the time, particularly during some crucial moments during the Seven Days' Battles and Antietam, definitely influenced McClellan towards blowing some opportunities to pin Lee and, by extension, win the war.