I think people have delusions of how big Coventry is.
The Coventry Urban area has a population of 360k. The Leicester urban area has a population of 508k. The two cities are not comparable.
The urban area figures you are quoting are out of date. Irrespective, team support isn’t based purely on the built-up conurbation of a city or town, surely?
As an aside, the urban population stat for Leicester includes Wigston and Oadby which isn’t too dissimilar to Coventry if Bedworth and Nuneaton were included in the urban populace.
Also, Exhall is included in the built-up area for Coventry but Bedworth isn’t despite being connected to Exhall. Binley Woods is included despite being practically no closer to Coventry than Kenilworth. Millison’s Wood is excluded despite being historically part of Allesley and although considered part of Meriden is almost as close to Eastern Green as it is to Meriden.
It’s all very muddy, you might as well stick a finger in the air and say the potential support for Leicester is the City and County of Leicestershire and for Coventry it’s Coventry and Warwickshire despite both having supporters worldwide.