There's very likely to be different models and scales for scarabs
In first strike it mentions spider diggers (or something) but they were blown up by a single wraith shot to each
Also, the new scarab is alot more combat orientated, the scarabs in halo 2 were more or less purpose built mining machines, because the covenant didn't expect a fight when they came to earth
The one in halo 3 has a secondary main turret, which points towards it being built for combat
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