CZ-5-300 F9 First stage length (m) 27.6 42.6 diam. (m) 3.35 3.66 gross mass (kt) 156. 433. thrust (SL, kN) 2400. 7607. burn time (s) 180. 162. Impulse/volume (kt/s/m^2) 1776 2750
Both use RP-1/LOX which is not as unusual for a side booster as I originally thought.
If I very crudely estimate mass-specific impulse and volume specific impulse for these using sea level thrust times burn time I get
CZ-5-300 F9 First stage comments Impulse/mass (m/s) 2770. 2846. similar Impulse/volume (kt/s/m^2) 1776. 2750. big difference
Their sea level Isp's are similar, but the CZ-5-300 has only 2/3 the impulse per unit volume. Is there anything meaningful to this difference (extra stuff inside) or is this likely trivially explained or otherwise unimportant?