according to my experience after more than 2 decades in aviation , there is no problem to go from a small/medium to a wide body as long as the training given is good and long enough and that the trainee has the capacity it takes.
There were/are Airlines where the cadets are going directly to the wide bodies as those companies don't have anything else , of course they required long and thorough training to achieve that goal and not all of the candidates will succeed.
In my career I went from a B737-200 first officer position to a MD-11 first officer and this transition has been done without any problem but I have had a very good training.
I think that what is more important is what kind of initial training you have had , that is where you get the main part of your basic skills and gain good habits and as long as you keep it , the rest of your career will just be an easy flow.
As a trainer I usually have had less difficulties to train cadets without any experience than direct entry pilots with , most probably , less than good initial training and sometimes attitudes making their training more difficult.
I can even tell you that I have seen a DC-10 captain struggling to learn to fly a glider at the time I was a glider instructor…
So it is wrong to say that flying a wide body is more difficult to fly than anything else , the truth is that each " flying machine " as its own difficulties that should be learned and as long as your are properly trained and you have the capacity , there will be no problem.