The idea is wrong that meditation is ‘expanding the mind’ while ‘concentration is narrowing the mind’ and that the two are mutually exclusive.
The word ‘mindfulness’ (‘sati’) in Buddhism means to ‘remember to apply’ or ‘keep in mind’ the Buddhist path; which means to keep the mind in a clear & aware state that is free from unwholesome & unbeneficial mental states, such as craving & distracting thoughts.
The word for ‘concentration’ in Buddhism is ‘samadhi’. The word ‘samadhi’ means ‘collectedness’ & refers to when the mind gathers together its energy, power, awareness, wisdom & other wholesome qualities due to its practise of ‘mindfulness’. The Pali scriptures states:
Any singleness of mind equipped with these seven factors — right view, right resolve, right speech, right action, right livelihood, right effort & right mindfulness — is called noble right concentration with its supports & requisite conditions. MN 117
When the mind achieves the perfection of ‘samadhi’, this is called ‘one-pointedness’ or ‘jhana’. The mind here is completely exalted & expanded, i.e., ‘heavenly’. The term ‘one-pointedness’ does not refer to a ‘narrow’ awareness. Instead, it refers to a mind that is completely undistracted.
An analogy is a wheel that spins around an axle. The wheel is fixed to & revolves around the axle. This being fixed to the axle is 'one-pointedness’. However, the wheel itself, which is an analogy for the mind, is not ‘narrow’. Instead, the wheel/mind is both expanded & perfectly functional. If the wheel is not fixed to the wheel, the wheel cannot function properly.
In summary, the proper practise of ‘mindfulness’ causes ‘concentration’ to happen. In other words, ‘concentration’ is continuous unbroken ‘mindfulness’. The Buddhist scriptures state:
In one of right mindfulness, right concentration comes into being…