Fighting

We have two Russian Tortoise's. We have had one for almost two years, the other one year. Our youngest one has been growing by the day and is very active - the other one has not grown at all and really doesn't move around alot(not sure if this has anything to do with it) - but last night we noticed that the little one was very aggressive and kept trying to fight the big one. What is going on all of a sudden and should they be separated?