Hannibal Lecter is not a leading character

Lecter is because he drives the story despite his limited screen time.

You're taking "leading character" too literally.

There is physical presence seen on celluloid counted in frames and seconds and then there is psychological presence, along with interplay. Lecter is a narrative / directorial conceit.

Not only does he drive the story forward as another poster mentioned, but his relationship with Starling helps us draw out the character makeup of both (as opposed to personality.) The last and next visit to Lecter is always on Starling's mind - and by extension, our minds.

Hopkins / Lecter doesn't stand around wasting time and frames, either.. coasting on movie star charisma in some biographical drama playing a historical figure we are somewhat familiar with. When you play, say, Stephen Hawking, I'm distracted by how Stephen Hawking-like your mannerisms are. It doesn't strike me to notice if the scene you were in lasted 30 or 50 seconds.

Lecter is a work of pure fiction. Hopkins, at times, savagely attacks the role with an economy of expression. Early on when we see him, before he even utters a word, he is just standing in his cell and perfectly lit; oh, but the way he is standing - all coiled with pent up energy, like a cobra / rattlesnake / serpent ready to strike (Thanks to Ebert for that vivid image.)

~17 more minutes of spraying this napalm on-screen is a reasonable amount of time to make a play for "Best Actor" instead of "Supporting," - without falling under a threshold and becoming ridiculous.

It's a testament to Demme's pacing / editing, and of course, to Hopkins' acting prowess. That he was able to project such forcefulness and dominance into a creation that - without a stopwatch timer to measure - seemed to pervade the air of the movie for much longer than that, is no small feat.

At times during my first viewing, I almost forgot that the "primary antagonist," and "villain" was supposed to be Buffalo Bill / Jame Gumb. Even if Gumb were to theoretically appear onscreen for ~45 minutes, almost 3x as long - I would probably still find Hopkins' Lecter more psychologically compelling and central to the movie.

I cannot take anyone seriously who refuses to accept that Foster & Hopkins are, without a doubt, co-leads of this movie - while the others just fall into the metaphorical out-of-focus zone in the background. The real story is the story of Starling and Lecter, and the rest are mostly pushing along a surface-level plot that has to run its course.

The always watching and listening Hal 9000 is barely "seen" in most of 2001: A Space Odyssey yet it is arguably as important a character to the movie as the leading character Dave Bowman. The movie's themes and broad scope would not be the same without their interactions.