The idea that big firms don't get sufficiently punished (or seem to attract that much attention) isn't correct. In terms of maintenance, First had issues in Bristol that led to a new MD, fleet investment and improved systems that massively improved their record. Similarly, Stagecoach in Mansfield also got caught by the TC's about 10 years ago and had a license cut.
Big firms take it very seriously and, as a transport manager, CPC and O license holder myself in the past, I can tell you that no manager wants to get their business in front of the TC. Not least because any manager may end up getting dismissed, as happened with the Stagecoach example, and also lose your good repute. One of my old bosses used to get his managers (of which I was one) to actually go and "mark each others' homework" by doing an annual check that we were legally compliant, and that was in addition to the official internal checks within the company. I've also had the "man from the ministry" visit and it isn't a nice experience.
Also, large firms invest a lot of time and money in training their staff, investing in technology and their fleets. Small firms just don't have that infrastructure. When they're very small, they can exercise a level of control but when they grow to having 15+ vehicles and outbases, that's when small businesses can overeach themselves by not being able to control their maintenance and legal compliance effectively.
And, of course, you always have the problems of rogue individuals (as in any trade) who are either incompetent and downright dishonest. I'd point out that the TCs are very short of staff from budgetary constraints so they have to be selective and only go for particular cases, either the worst or the repeat offenders (of which Arriva would seem to fall into the second).
TC's and the OFT used to work together in respect of individual routes such as Badgerline and Northumbria's issues. Where compliance was the issue, it's the TC. Where it was predatory behaviour, it was the OFT then CC.
