Having a lot of plastics around the world who know nothing about the history of the club, and have a passing interest in the club and football in general, is a much less important and substantial factor than what you have won, the quality of players you have had in your history.
Frankly, Man United's trophy cabinet and player quality lags far behind AC Milan, Bayern and especially Barcelona and Real Madrid. Barcelona and Real have had many of the best players of all time play for them, whereas the best players ever for Man United are Duncan Edwards, Cristiano Ronaldo, Bobby Charlton and George Best. Excellent players no doubt, but don't compare to legends from the Clasico sides. Though a large part of this is that Latin footballers (who are more attracted to La Liga) are generally far better than British and Irish players (where United traditionally source from).
United have won three European Cups, which is five times less than Real Madrid and not very good compared to other clubs too. They have won 20 league titles, but 13 of those came in the PL era. Their success has generally been in two spells in their history - under Busby and Ferguson. The only manager ever to win a league title for them outside those two is Ernest Magnall. They also almost went out of business a few times, and if people realised football didn't begin in 1992, they'd know Man City were probably the biggest club in Manchester until the 50s and even let United play at Maine Road when Old Trafford was bombed in the war. United made their name off the back of a tragedy (the Munich air crash) and are still dining out on it til this day/
There's other reasons why United (and Liverpool) are not on the level of Real Madrid and Barcelona (for Liverpool it's because they generally have good but not all-time legends and don't play as entertaining football as Barca).
On all metrics - trophies, legendary player list, stadium (now Barca are building a new one) and various other metrics, Real Madrid and Barcelona are the two biggest clubs in the world, in that order. I would say Bayern are next, then Liverpool and United. Juventus, Milan and Ajax would be higher but have fallen in recent times due to being hampered by the decline of the leagues they play in.