Saving Jpegs from Smart Objects [file size]

Hello guys! I've been using Photoshop CC 2017 version with no trouble before. Recently, I placed 3 layered images, converted it to smart objects and placed it in a new document. When client asked to have separate jpegs of the three, I opened each smart object, saved a flat copy, converted to RGB then saved each as jpegs. The size of the jpegs are 21.9 MB, 19.4 MB and 2.2 MB. Before, the sizes of these jpegs when I was saving them as SEPARATE psd files were only 1.7 MB,  1.5 MB and 2.2 MB. Did something happen when I converted them to smart objects? Why would they save as large jpeg files when they are all flattened, RGB and all of the same size? Why did the third jpeg remain the same file size when the others did not?