If it seems like I've pointed out a lot here, well...this sort of thing is a problem for me too, knowing I have a space and development advantage I should be able to cash in but I don't. So I ran this through Fritz to see improvements. And you may like to do so, too. Sometimes it's frustrating to see a move that looks good but you don't quite have the guts/justification to play it.
Fritz gives 8. e5 Nd5 9. a3. It's always tough to know when to push the pawn, and while 8. Nbd2 looks natural, the Black knight is a target for the pawns, especially with the pawn on b6. It can't go to b5. So Black has to play ...Bg5 or ...g5 to protect it, each of which is ugly. ...c5 is a possibility, but dxc5 and Rd1 give you the open file.
With 8. Nbd2, an e5/a3 thrust leaves you open to Nf4. The thing is that while developing moves are good, the knight might be better placed on c3, and a3/c4 might gain space and give you that square. Also, if you are worried c3-c4 is wasting a move, remember that's not really the case. ...Bb4+ c3 Be7 provoked it.
Nxe5 also seems to relieve the pressure on Black. As a general principle you shouldn't exchange pieces when you have a space advantage unless you get something in return. But you still have an edge. Another poster noted the Bb7 could gain scope, but with a knight or bishop on e4, you are more likely to block it.
... c6 is also weak. It blocks the bishop, and c4! is vigorous. ...Nc7 is awkward, but ...Nf4 14. Qf3! loses a pawn. ...Nb4 15. a3 Na6 16. Qg4 and Black must play ...g5 or ...g6 with kingside pawn problems. White will again get the d-file. You played c4/a3 in the game, but letting Black get ...a5 in meant you found it tougher to play b4 and restrict Black's pieces on both sides of the board.
Finally, this is something I had to throw into an engine, but Rd3, while it doubles the rooks, also lets Black exchange them off. Fritz's idea of Nd2 followed by Qg4 and Ne4 looks strong. The point is that Black is too busy defeinding a kingside attack to swap rooks right away.
- Bf4 also seems more accurate than Be3. It allows for an attack with Bc2 and Qd3 or e4. Black's having played ...h6 makes for a potentially weak kingside.
It looks like you considered the moves that a computer engine marked off as the best, and in some cases you played them a bit too late, and in some, a bit too soon. When you have more space it's tough to know when to squeeze and when to use tactics. Often you don't need to "just" develop with a space advantage. It may be better to restrict your opponent from meaningful development, for instance, with the early 8. e5. It's good to check for overextending, but Black's development isn't good enough to poke at any of your weaknesses.
The main problem for White is to figure how to take advantage of space and make Black's pieces stumble over each other. The Bb7 is rather good, but once Black plays ...c6 or ...c5 there are is that weakness on d6, and e4 is always available for a bishop or knight.
