As Steven is the same height as he's been since he was 8, i'll be using this chart, which makes him 135 cm.
1.35 m
1.35 / 334 = 0.004 m/px
0.004 x 106 = 0.424 m (Upper Body Height)
0.004 x 108 = 0.432 m / 2 = 0.216 m (Upper Body Upper Radius)
0.004 x 257 = 1.028 m / 2 = 0.514 m (Upper Body Lower Radius)
Upper Body = 171175 cm^3
0.004 x 276 = 1.104 m (Lower Body Height)
0.004 x 257 = 1.028 m / 2 = 0.514 m (Body Radius)
Lower Body = 916315 cm^3
1.94 - 1.516 = 0.424 (Head Height)
0.004 x 108 = 0.432 m / 2 = 0.216 m (Head Radius)
Head = 62000 cm^3
Density of obsidian is 2.6 g/cm^3
171175 + 916315 + 62000 = 1149490 cm^3
1149490 x 2.6 = 2988674 g = 2988.674 kg, Class 5