Is it sometime advantageous to split a transaction into multiple smaller ones? When I updated Bitcoin Wallet on my Android device, it automatically created a new address and transfered all my funds from my old address to it due to the recently found security issue in Android's random number generator. 

However, it didn't do it with a single transaction. Instead it did two transactions, so I had to pay the transaction fee (0.0001) two times. 

Did it perhaps split it into two smaller transactions to keep the size down (only a guess) for the sake of the network? Or is a smaller transaction processed faster? Or was this necessary for completely emptying the old address? Or was my old address fragmented after multiple transactions to and from it in its history (if that is even a thing/possible)? 

My questions above are only speculation. And my real question is: is it sometime advantageous to do two (or more transactions) to the same destination instead of a single, big one?
It can be advantageous to split transactions in order not to draw from different addresses in one transaction, because that would announce that both addresses belonged to the same Wallet (for the privacy paranoid user).

The priority of a transaction is reduced by having more source addresses, however it is on the other hand increased by sending a larger amount. Coins that have been held for a longer time also increase the priority.

I don't see why it would have been necessary to transact in this fashion and I think the overall size for the network would be the same (2*250B or 1*500B).

I would assume that you had two different addresses holding coin that were each emptied each in one transaction.