Liverpool are this week celebrating winning the Premier League title while United languish in the bottom half of the table.
Speaking on his Rio Ferdinand Presents YouTube channel, he said: "They're (Liverpool) the most successful club in the land, but they're not the biggest club.
"It's a fact. You go anywhere in the world, there are more Man United fans than there are Liverpool fans. But that will change if they remain successful and Man United don't. There will be a swing of new fans who adopt successful teams.
"If Liverpool become that over the next 10 years then they have a chance to balance that up, in terms of the biggest club in the world right now. But right now it's a landslide win for Man United in terms of being the biggest in the world."
On United's struggles in comparison to Liverpool's success, Ferdinand conceded: "It fries my head. I've got to be honest, when I was at Man United, we were miles ahead, won leagues, etcetera.
"We clawed it back and then were ahead. I was part of that and it felt great. And now I'm just sitting and watching it before my eyes, just all sliding away. Liverpool have become more successful in the time since I retired. You've just got to accept it."