If we're talking about "size" as in geometry (i.e. number of pixel elements), then if the YCbCr is not downsampled, then the image channels have the same number of elements before and after conversion. If the YCbCr image is subject to chroma subsampling, then there are fewer elements in the Cb and Cr channels. After conversion to RGB, the image will have more elements than it did when it was YCbCr.
If we're talking about "size" as in memory, then it depends on how it was converted, and again, whether chroma subsampling was used. If no subsampling is used and the same numeric class is used, then the two images will have the same size in memory.
Argb = imread('peppers.png');
Aycc = rgb2ycbcr(Argb); % this is uint8 with no subsampling
Argb2 = ycbcr2rgb(Aycc); % this is also uint8
whos Aycc
whos Argb2
If we're talking about images in files, I think it's safe to say that the most common numeric class for either type is uint8, though the varied methods of compression across different image formats kind of makes the comparison moot.
TL;DR: The channels of an RGB image are represented with equal spatial resolution, but the channels of a YCbCr image may have unequal spatial resolution.
If you're referring to some other aspect of the images, you'll have to clarify.
