There is more than one version of that particular phone. They come in 32, 64 and 128 GB versions. So it is going to depend on which version you have.
If this is a newer phone and it isn't matching the info the IMEI is pulling up (although that can be off depending on where you are getting the IMEI info from, not all places give good info) then I would contact the place you got it from and see what the phone as advertised as having. If it was supposed to have more, per their advertising or the information on the box the phone came in, then they need to get you a phone with that correct amount.
However, if you are only going by wherever you got the data from, then since the data may not be correct, you may be stuck with a phone with less.
Specs on that phone... http
/www.gsmarena.com/samsung_galaxy_s6-6849.php