May be I miss something obvious but could you please give a hint how to check the size of all content of Smart Folder?
Select all the files in the smart folder. Then hold down the Option key and do Get Info. That will open a single window summing the data fromall the files instead of one window per file.
Select all the files in the smart folder, hold down the Control key, and right-click to select the "Get Summary Info" option. It will show you the total size for the selected files.