One of the unalterable truths of life is that some coppers are bent. Another is that bureaucracies will tend towards secrecy, hiding things, anti-democracy and unlawful behaviour. The larger and more complex the organisation, and the less the organisation is subject to public oversight and control, the greater the chances of both bent coppers and bent commanders.
Conversely, small police forces subject to a high degree of local oversight and scrutiny will have few places for bent coppers and police bosses nowhere to hide their maladministration.
Which is why it's hard to explain why ACPO are pushing so hard for mergers to create ever larger, more complex and remote police forces in which low level corruption will flourish and commanders are able to hide, deceive and obfuscate unlawful and improper behaviour unless they are actually trying to promote such things.