For some reason, they make it very difficult to increase the memory size of SQLDeveloper, you can't even pass in the options on the command line.
I'm not sure about Mac or Windows, but on Linux, it's pretty easy, just edit your ide/bin/ide.conf file and change the lines "AddVMOption -Xmx640M" and "AddVMOption -Xms128M" lines to something larger (or smaller). You can see what I have mine set to below.
Here is mine after I changed it:
tor@XXX:~/sqldeveloper$ pwd
/home/tor/sqldeveloper
tor@XXX:~/sqldeveloper$ cat ./ide/bin/ide.conf
#-----------------------------------------------------------------------------
#
# Oracle IDE Configuration File
# Copyright 2000-2008 Oracle Corporation.
# All Rights Reserved.
#
#-----------------------------------------------------------------------------
IncludeConfFile ../../ide/bin/jdk.conf
AddJavaLibFile ../../ide/lib/ide-boot.jar
SetMainClass oracle.ide.boot.Launcher
#
# If you are getting the 'Low Memory Warning' Message Dialog while running
# JDeveloper, please increase the -Xmx value below from the default 768M to
# something greater, like 1024M or 1250M. If after increasing the value,
# JDeveloper is no longer starting up because it fails to create a virtual
# machine, then please reduce the modified -Xmx value.
#
AddVMOption -Xmx2048M
AddVMOption -Xms512M
#
# Turn off verifications since the included classes are already verified
# by the compiler. This will reduce startup time significantly. On
# some Linux Systems, using -Xverify:none will cause a SIGABRT, if you
# get this, try removing this option.
#
AddVMOption -Xverify:none
#
# On some Windows Terminal Server installations, relocation errors of
# system DLLs can sometimes occur when using OJVM. Use this option to
# specify a heap base address for OJVM to resolve this problem. This
# option only works with OJVM and cannot be used with any HotSpot JVM.
#
# AddVMOption -Xheapbase100000000
