Krishna says, "Among the 𝘼𝙙𝙞𝙩𝙮𝙖𝙨 , I am Vishnu; I am radiating Sun and Moon (10.21). Among the Vedas, I am 𝙎𝙖𝙢𝙖 𝙑𝙚𝙙𝙖 ; I am 𝙑𝙖𝙨𝙪𝙫𝙖 (𝙄𝙣𝙙𝙧𝙖) ; among the senses, I am Mind ( 𝙢𝙖𝙣𝙖𝙨 ); in creatures, I am Chetana (consciousness) (10.22). Of the 𝙍𝙪𝙙𝙧𝙖𝙨 I am 𝙎𝙝𝙖𝙣𝙠𝙖𝙧𝙖 ; I am 𝙆𝙪𝙗𝙚𝙧𝙖 (10.23). I am 𝘽𝙧𝙞𝙝𝙖𝙨𝙥𝙖𝙩𝙞 ; among expanses of water, I am the ocean (10.24).
Krishna earlier said that HE is the 𝙖𝙩𝙢𝙖 (soul) in the heart of all beings (10.20) and gave a hierarchy that the 𝙖𝙩𝙢𝙖 is superior to 𝙗𝙪𝙙𝙙𝙝𝙞 (intellect); 𝙗𝙪𝙙𝙙𝙝𝙞 is superior to the mind; the mind is superior to the senses (3.42). But here he says HE is the mind among senses and this needs understanding.
It is said that 'the whole is greater than the sum of the parts'. As a whole two ears can bring a sense of direction of sound; two eyes together can produce depth perception. One way of looking at the mind is as a simple sum of its senses like ears, eyes etc. Another way is to look at it 'as a whole of senses' where together these senses can perceive far beyond what a sum of all senses can do. This is also known as 'beyond senses' or 'sixth sense' in the modern context. Krishna was referring to this sixth sense when he says HE is the mind among senses.
Krishna further says, "Among words, I am the 𝘼𝙪𝙢 (𝙊𝙢) ; among stationary objects, I am the Himalayas (10.25). Among all trees, I am the peepal tree; I am Narada; I am the 𝙢𝙪𝙣𝙞 𝙆𝙖𝙥𝙞𝙡𝙖 (10.26). I am 𝘼𝙞𝙧𝙖𝙫𝙖𝙩𝙖 amongst all lordly elephants, and the king amongst humans" (10.27).
One common thread in all of the above is that out of the available options HE represents the greatest possibility of 'peak performance' like the water becoming the ocean. options HE represents the greatest possibility of 'peak performance' like the water becoming the ocean.