Spread it!
When you travel to Disney, you will buy souvenirs. That's just a fact. Even the most disciplined traveler will buy something from the parks, resorts, or Downtown Disney.
Ultimately, this means you will take home more than you brought to Orlando. Act like the Boy Scouts and "Be Prepared!" Bring an extra suitcase to Disney.
With expanded carry-on and checked luggage restrictions, you may consider using packing cubes as your additional bag, since these will flatten and fold inside your current luggage. This way, you may only need to pay an extra bag fee for the return trip.