The Warbird doesn't need anymore engine-space than any of the other DN's
Maxed out DN's, all the Shield, Weapons, Hull and Bridge mass used. On top of that a Warp core X (10)
Engine-mass left after warp core X installed
Galaxy ------- 1300
Sovereign ---- 2300
Negh'var ----- 2500
Warbird ------ 1850
Sphere prime - 2800
Now you are gonna say "Ha see, the Warbird needs more"
Well the Warbirds age is the same as a Galaxy, where both the Sovereign and the Negh'var are of a much newer date, the Sphere who knows, so looking at that I would say the mass seem fine.
Just a few more numbers, all ships as before, only thing added is a legendary crew.
Galaxy Thruster III + Impulse III -- Speed 17,31 Turn 0,42 Mass 47750
Sovereign Thruster V Impulse V -- Speed 21,81 Turn 0,48 mass 53700
Negh'var Thruster VI Impulse V -- Speed 28,36 Turn 0,70 Mass 52850
Warbird Thruster V Impulse V ---- Speed 26,37 Turn 0,56 Mass 47750
Sphere P. Thruster VI Impulse VI - Speed 37,81 Turn 0,76 Mass 33350
Let me guess now, what you are gonna say, "see it's about 6000 short in mass compared with the Sovereign or Negh'ver..."
Well Romulan shield X is 1500 less in mass than the same Federation or Klingon shield X, but at the same time it has same strenght and power need as the Klingons shield X, now add 1500 with 4.
Also adding to this is Romulan thrusters have less mass, but more power than Federation thrusters, Klingon thrusters lead the way. Impulse are almost the same as with thrusters, only that here Romulan lead the way with low mass, high output.
Now with all this said... do the Warbird still need more engine-mass ???