What version of weblogic are you using? By default, if you use the startManagedWebLogic script, it will use the values defined in the setDomainEnv script, but if you use the node manager to start them, then it will use the values in the Startup settings instead, but that can be overwritten in the node manager config, but assuming you have not changed those.
And btw, 800MB-1.1GB isn't a lot of memory these days and will be required if you load FMW applications into weblogic.
If you use weblogic 12c, then it is adviced not to change the setDomainEnv.sh, but instead create a setUserOverrides.sh in the same folder and set user settings over there.
Then indeed as handat says, there might be a difference when starting from nodemanager or using the start scripts. However, in the nodemanager.properties you can set properties to use the start and stop scripts (what I would recommend).
But, it also depends on the java version. You did not set the -XX:MaxPermSize (Java 7 and prior) or -XX:MaxMetaspaceSize (Java 8 and above). So that adds to the OS process.
But, not only that, Java VM itself also needs a bit of OS memory to work. For internal algorithms etc. So if you have a heap of max 256M and a Max PermGen/MetaSpace of 256M, you would conclude that the process size is 512M, But this could easily be 20% higher. So it would not suprise me you would see 800M.
Also, Weblogic uses native libraries for IO. These are natively compiled C libararies that are also loaded into the native memory part of the JVM process.
Then when not setting a maxMetaspaceSize (Java 8) there is no limitation. Java will managed this itself.
But also, earlier this year we've seen with FusionMiddleWare including WebServicesManager the AdminServer would grow beyond the maxMetaspaceSize, due to excessive load of duplicate classes (this was a bug).
Thus the story is a bit more complex then adding heap + metaspace/permgen size.
Check the parameters of managed server defined in server start section from console and make appropriate changes to memory parameters there as well.
Did you solve this already? If not, run JVisualVM or jmc from the java home on your system. Then open the particular jvm that runs your weblogic server.
On the System properties tab (or similar) you can investigate the startup parameters of your JVM. Also on the memory tab you can see the current memory consumption.
You could also investigate the server.log and/or the server.out file. There you can also investigate the classpath and memory settings. These are a result of the conditions in the startup scripts. Probably you changed settings in a part of your scripting that did not get touched because of the conditions.
Do you use NodeManager to start your server? Or do you use the startup scripts?
Also, can you show a screendump of the servers tab of the weblogic console?