How is the tune adjusted for bigger injectors?

By entering the new injectors' data, then confirming with logs.

• Flow rate is the foundation of the fuel calculation. If it's wrong, the engine never runs right, or the error hides a mechanical problem (LSX Magazine). HP Academy puts injector setup among the first steps of any tune.
• Enter the full characterization, not just flow. In LSX Magazine's 1,000cc injector swap, the tuner entered the supplied flow rate, offset (dead time) and short-pulse adder data.
• Match the fuel pressure. Injectors are rated at a set pressure, commonly 43.5 psi (3 bar) or 58 psi (4 bar), and flow scales with the square root of the pressure difference. Set the fuel-pressure reference correctly (atmospheric vs manifold-referenced), because flow and dead time both change with differential pressure.
• Use characterized injectors. Summit's LS guides recommend fully characterized injectors so the tuner can get idle and drivability right.

Then confirm with logs: fuel trims and wideband readings at idle, cruise and WOT.

Want to do it yourself? See who tunes it, and how to learn.