You can find the solution at the FAQ section of this Website:

How can I adjust the memory used by Sweet Home 3D?
Sweet Home 3D can't use as much as memory as your computer can offer by itself, but you can still change the maximum memory used by the program if needed. This may be required for example if an error red cross appears during the photo rendering of a home with many different and detailled 3D models. By default, the Java Web Start version uses 640 MB or 2 GB at maximum according to your system, and the installer version uses 1 GB or 2 GB at maximum, except under 32-bit Windows where it's still 640 MB (this maximum is displayed in the About dialog box). For the installer version, you can change this number by udpating the value of the -Xmx Java option, as follows:

• Under Windows, create a text file named SweetHome3D.l4j.ini in the same folder as SweetHome3D.exe, write the maximum memory Java option in that file, for example -Xmx700m for 700 MB, then relaunch Sweet Home 3D. For a value higher than 700 MB, tested as the highest value under which Sweet Home 3D can run safely on a 32-bit Windows computer with little RAM, you should use a computer running 64-bit Windows and install Sweet Home 3D 4.3 or higher.
• With the free version under Mac OS X, click on the application icon of Sweet Home 3D while maintaining the ctrl key pressed to display its contextual menu, choose Show Package Contents item, open the Contents folder, edit the Info.plist file by dragging its icon on TextEdit application and update the maximum memory in the text <string>-Xmx1024m</string>, writing for example <string>-Xmx2g</string> for 2 GB instead of 1024 MB. Finally, save the file and relaunch Sweet Home 3D. If Info.plist is protected, you may run a command like sudo nano /Applications/Sweet\ Home\ 3D.app/Contents/Info.plist in a Terminal window to edit it (this command requires to enter the password of the administrator of the Mac).
• With the Mac App Store version, edit the SweetHome3D.cfg file found in the Contents/Java subfolder of Sweet Home 3D application and update the maximum memory by adding the option -Xmx after the line [JVMUserOptions], writing for example -Xmx6g for 6 GB. Save the file and relaunch Sweet Home 3D. To edit the SweetHome3D.cfg protected file, you may run a command like sudo nano /Applications/Sweet\ Home\ 3D.app/Contents/Java/SweetHome3D.cfg in a Terminal window (this command requires to enter the password of the administrator of the Mac).
• Under Linux, edit SweetHome3D script, update the maximum memory in the option -Xmx1024m (using for example -Xmx2g for 2 GB), save the script, then relaunch Sweet Home 3D.