How to use [saveas function] for figure to PDF in original size(real size)?

How to use [saveas function] for figure to PDF in original size(real size)? image size is 2832*4256*3 unit8. but the saved file(.pdf) is only 326KB.

figure(); subplot(6,3,1), imshow(RGB), title('RGB'); subplot(6,3,2), imshow(gray), title('Gray');

subplot(6,3,4), imshow(red), title('Red'); subplot(6,3,5), imshow(green), title('Green'); subplot(6,3,6), imshow(blue), title('Blue');

subplot(6,3,7), imshow(just_red), title('Just Red'); subplot(6,3,8), imshow(just_green), title('Just Green'); subplot(6,3,9), imshow(just_blue), title('Just Blue');

subplot(6,3,10), imshow(graytosobel), title('Gray(Sobel)'); subplot(6,3,11), imshow(graytocanny), title('Gray(Canny)'); subplot(6,3,12), imshow(graytoprewitt), title('Gray(Prewitt)');

subplot(6,3,13), imshow(redtosobel), title('Red(Sobel)'); subplot(6,3,14), imshow(redtocanny), title('Red(Canny)'); subplot(6,3,15), imshow(redtoprewitt), title('Red(Prewitt)');

subplot(6,3,16), imshow(Justredtosobel), title('Just Red(Sobel)'); subplot(6,3,17), imshow(Justredtocanny), title('Just Red(Canny)'); subplot(6,3,18), imshow(Justredtoprewitt), title('Just Red(Prewitt)');

saveas(gcf, datestr(now,'yyyy-mm-dd HH-MM-SS-FFF'), 'pdf')

[Jan]

What is "real size"? In bytes, or pixels, or centimeter? PDF can contain compressed JPGs. If so, the size of the file depends on the chosen resolution and strength of compression also. So what is the actual problem?