As I far know 15MB is the limit, then you have to use Google drive, mediafire to upload and then give the link to the map

Well they need a size limit because planetMC has hundreds of thousands of maps, if they did not have a size limit, they'd run out of storage(or have slower uploading speed due to all the large files being uploaded). Best bet is that you use an external file service(where you can upload your world) then put the download link to your world in your planetMC map page, and your good to go!

There are most likely tutorials on how to do this throughout the internet.