At some point I heard you can gain about 0.5 lb of fat per week. Like actually have your body store and establish that as part of its composition. Additional fluctuation in weight is going to be primarily due to water, but in consistently overfeeding yourself or nutritoinally starving yourself, you are setting up your metabolism to slow down its burn rate - so you might not be gaining a discrete amount of fat in a given day, but rather just establishing a lower potential for building muscle / burning fat or generally efficiently metabolizing the stuff you eat.