On Windows there is an
igv.bat batch file that you must be using to start IGV. You could edit this line
jre1.8.0_NNN\bin\javaw -Xmx2G -Dproduction=true -Djava.net.preferIPv4Stack=true -Dsun.java2d.noddraw=true -jar %BatchPath%lib\igv.jar %*
and increase the
-Xmx parameter to a higher value
-Xmx6G. Since you only have 8G of RAM for windows you don't want to go any higher. If this does not suffice then you need to find hardware with better specs.