Why is it that when I first wake up my muscles (usually the arms (biceps & triceps) look slightly smaller? then maybe 3 to 4 hours in the day they began to look normal size and more muscular ?

Probably because there is less blood flowing through them due to them not being used for several hours whilst you are sleeping.

This is why, after weight training, you often get a 'pumped' look because your muscles are more full of blood due to the intensive workout you've done. I love the way I look after the gym, shame it fades away!

It's perfectly normal though.