This is an interesting question that pops up on Google when you search for information about Jacksonville. I believe people want to know the most populous or populated city in Florida, unless they are total tourists who are just interested in popularity. As far as being popular, I suppose Miami is tops, then Orlando and Tampa. Population is always a bit more complicated.
By metro area, here are Florida's most populous (highest population) along with 2018 US rank:
#7 Miami-Fort Lauderdale-Pompano Beach, FL MSA
#18 Tampa-St. Petersburg-Clearwater, FL MSA
#22 Orlando-Kissimmee-Sanford, FL MSA
#40 Jacksonville, FL MSA
#72 North Port-Sarasota-Bradenton, FL MSA
#76 Cape Coral-Fort Myers, FL MSA
#81 Lakeland-Winter Haven, FL MSA
#87 Deltona-Daytona Beach-Ormond Beach, FL MSA
#95 Palm Bay-Melbourne-Titusville, FL MSA
That's top 100 and over 500,000 population, anyhow.
Here are the cities (not metro areas) in Florida, ranked by most populous within the city limits:
#79 St. Petersburg
Keep in mind that Jacksonville's area is 20 times that of Miami. So, Jacksonville is the largest city by area, and largest city by population within its city limits in Florida.
Side note: Rhode Island, with a population just over one million, is only about 40% larger than the CITY of Jacksonville in area. That's crazy. And a good reminder why there's no city of Jacksonville walking tour map.