Bay must be getting more lenient about these things, especially considering Optimus isn't the only offender. Lockdown, for instance, transforms into from a rather massive, bulky mech into a car that doesn't quite look big enough to fit his mass.
Such things have been part of Transformers since "More than Meets the Eye, Part 1" first aired. I usually find its best not to think about such things; saves me a headache. :p