That 30,000 number is for Alexander exiting India, I imagine it was somewhat larger embarking on the campaign. I'm not sure we should judge the army's effectiveness by its size, Alexander had an elite veteran core that could basically overcome any obstacle within reason. Porus does not look to have very many men himself, Arrian says he numbered only 34,000. We also should remember that Alexander and his army did, in the end, turn back when it became unreasonable to continue.How is Alexander defeating the great Indian armies such as that of Porus and completing sieges like that of the Mallian capital with just 30,000 men? The distance between Persepolis and northern India is about the same as the distance between Persepolis and Greece. Alexander had extensive time to prepare for the campaign in India. He wouldn't have advanced so deep into India with just 30,000 men in my opinion.