Now don't get me wrong, Jeremy Clarkson, may have been a bit of a buffoon at times, and done and said some rather insensitive things, but at times he was actually quite insightful about some aspects of cars and motoring. If nothing else it may have caused some of us to think and re-evaluate our position on some motoring matters.
But in relation to "big is better" when it comes to engines he probably got it wrong, because that comment is based on an assumption that big old engines produce more power, and more torque. Fortunately both power and torque can be measured, and its the measurements that will tell the truth.
Where there may be an element of truth though is the usable power/torque range. Older engines tended to have flatter and wider power bands, whereas newer smaller engines may well have a much more peaky nature.
Is this a problem - well if you mate a newer style engine to an old style gearbox and transmission, it may well fail to deliver the beans because the gear ratios would be wrong, but a newer engine fitted to a properly matched gearbox should do a perfectly adequate job.
As for the reliability of new engines. Were have become pre-conditioned to expect new products to be unreliable, Caravans especially seem to be prime exponents of this syndrome and this was certainly true of older engines, and it is only due to the long production runs of some older designs eventually managed to iron out the problems they had.
Historically new products do tend to have a period just after introduction where some issues may cause problems, this is a well recognised part of the product life cycle. But with modern design and testing processes I would expect the product to be more closely matched to the job it is designed to do right from the outset.