when UE moves within TAs mentioned in TAL, UE will not trigger TAU since MME already know that UE can belong to any of the TA mentioned in the list. When UE moves to a tracking area which is not in the TAL. UE send TAU either UE in RRC-IDLE or RRC - connected.
There is tradeoff between TA size and frequency of TAU. So if TA size is too less then frequency of TAU increases. And when TA size is too large then MME will sends paging message to more number of eNodeB when UE is in RRC -IDLE mode and downlink data is coming for him from then core network.
So a wiser decision need to be taken with respect to TA size.