Are you referring to Version 1.* of S1TBX or to the SNAP 2.0 preview version?
For Version 1.* you can edit the file s1tbx.vmoptions, located in the installation directory.
You can increase the value for the Xmx parameter e.g. -Xmx5705M
The value is dependent on the amount of available RAM on your computer. By default it is set to 70%, if I remember correctly.
In Snap 2.0 you can look into the snap.conf file in the etc folder of the installation directory.
in the default_options line there is the parameter -J-Xmx.
You can increase this value to e.g. -J-Xmx5G
I think it has approximately the same default value as in S1TBX.
Marco