Why does image size increase after steganography?
I'll assume the question really is: when I perform low-order-bit steganography on an uncompressed image (inserting 6 bits encoding a character into the low-order bit of 6 bytes coding the R, G and B channels of 2 pixels), I find that compressing the resulting image gives a bigger file than compressing the original image for a given setting of the program used to compress.

The reason likely is that the process which made the original image is such that the low-order bits are very correlated with the other bits (for example this could be because of characteristics of a sensor; or because the original image has been compressed by some lossy algorithm, then decompressed); and that correlation is lost by replacing the low-order bits.

The observed effect will depend on the nature of the compression technique used, and will tend to be be most perceivable for lossless compression techniques (GIF, PNG, lossless JPEG), which are the only ones which allow correct decoding of the hidden message.

If the low-order bits of the original image where random, the observed effect would likely vanish, or even perhaps be inverted (assuming the enciphered message has redundancy).