Is it joke? No, Everton is bigger than Leicester in many ways.
Of course, result of assessment (in this case - which club is bigger) is depend from definition (what does it mean "bigger") and indicators of assessment.
For me, assessment "how big" football club is including:
1. Social dimension:
- support / fanbase (not only actual numbers of fans, but also loyalty, attendances during whole history, including bad times)
- structure of fanbase
- form and intensity of support (there is a huge difference between local fan with season ticket, attending every home and away match + buying souvenirs from official shop and foreigner "fan" never attending match, buying fake souvenirs from cheap shop and just following club on internet).
- fans economic impact (amount of money bringing by supporters / fans)
- attractiveness (for general football public, for sponsors, TV...).
2. History:
- history (whole history of club - football exist from 19th century),
- achievements (not only trophies),
- way how club achieved trophies,
- other things how club contributed significantly to football / sport.
3. Economic dimension:
- economic power - current, during history
- economic stability (it´s easy throw money for a few season for achieving something and then be relegated (Leeds etc.)
etc. This indicators also we have to divide to min. 2 groups - history and nowadays.
In most of indicators Everton are bigger club.