Changes in muscle size can be an illusion. Like said before, if you gain more fat, then you will look more smooth overall, and this can make your muscles look not quite as big, because they are not quite as defined. Also skin color (being tanned or not) can also make a difference in appearance.
A classic example of this is when a competitive bodybuilder steps on stage. He/she is tanned and muscles are super defined...they almost always LOOK bigger onstage because of this illusion...compared to when they are in the offseason, when they actually weigh more (are technically bigger), but may not look it.
Muscle glycogen stores,
water retention are also variables that can affect the way a muscle looks on a day-to-day basis, so keep that in mind also. But all in all, muscle gain or loss does not occur within a day or two...each happens over a period of time.