I use to have this problem with my old computer it keep just installing 32 bit and even if I got 64 bit installed it wouldent use it
how I SOLVED this was was I downloaded the 64bit JDK version ,when installed it gives you the Java Control Panel , should be installed to your start menu if not search for it .
in the java control panel click on he tab that says java and in there it will say view and manage version of java click on view ,make sure the current version of java that is 64bit is selected and enabled, If its not there (My problem) your have to Manually Add it to the users tab by clicking add and finding it in the java folder on your harddrive or when you installed it
after its selected is advised to do a reboot before running any java
after reboot run minecraft and it should be in 64bit
any question let me know ill try n help
*edit note, mixed up JRE and JDK you need the JDK version