The SPG-9 HE round has similar area of effect to a RGO grenade (~10m). The IndirectHit value of the HE round is identical to the HEAT round:

In real life, the SPG-9 projectile has about 2-4x the explosive filling of a hand grenade (~750g vs ~200g), as per wikipedia.

For other comparable projectiles/weapons, the indirect hit values are:
R_MRAAWS_HE_F = 6
R_TBG32V_F = 6
M_70mm_SAAMI = 6

Maybe the SPG-9's IndirectHit value should be increased to match the above recoilless rifles and RPGs?