Do oversize pulley wheels increase derailleur capacity? Do the large custom pulley wheels and cages increase the capacity of a derailleur significantly? There are reasonably priced ones on E-bay with 16t pulley wheels. 

Every demo of these I've seen says you'll need to install a longer chain when you switch, but does that really imply that the capacity of the derailleur has increased? 

I am mostly interested in this question because I'd like to run an 11-40 cassette on my gravel bike and use Campagnolo components if at all possible.
All else being equal, a longer cage will increase the capacity of the derailleur.
For example, the only difference in Shimano's long- and short-cage 105 5700 rear derailleurs is the cage.  See https://si.shimano.com/pdfs/ev/EV-RD-5700-3042A.pdf
Per https://si.shimano.com/pdfs/si/SI-5X90B-003-ENG.pdf that the short-cage SS models have a 33-tooth capacity while simply changing to a longer cage gives the GS model a 39-tooth capacity.
But note that the largest supported sprocket size remains unchanged, but that appears to be because the location of the jockey wheel closest to the sprockets also remains unchanged with the cage swap.
It also seems reasonable to assume that increasing the size of the jockey wheel closes to the sprockets without changing its location would result in a smaller maximum usable sprocket size.
I'd say you'll get more capacity, but you may not be able to use a larger cassette to take advantage of that larger capacity.
I also suspect that larger jockey wheels is likely to result in degraded shifting performance since the larger wheel could allow more side-to-side movement than a smaller wheel would.  &quot;But the marketing says it's stiffer and the bearings have tighter tolerances.&quot;  They would say that, and it might even be true.  When it's all clean and new and unworn - how durable and free-spinning will those ceramic bearings be when they get grit and grime inside:

“Ceramic bearings are beneficial in environments not requiring grease lubrication,” says Lew.  But a bicycle is expected to cope with a vast range of conditions, rain and dirt, and where maintenance schedules may be less than optimal, and the last thing you want is to ride bearings without grease. It’s this requirement to cope with the conditions common to cyclists that offset the promised lower rolling resistance of a ceramic bearing, according to Paul Lew.
“The rolling resistance of a ceramic bearing compared to an ABEC 3, 5 or 7 steel ball bearing is offset by the resistance of the grease,” he says.  “In order for a ceramic ball bearing to out-perform a steel ball bearing, grease is not an option. Does this mean I should run my ceramic ball bearings dry or with light oil? Yes, but you won’t like the result in an environment where the bearings can become contaminated.  If you run your bearings dry they will feel gritty and rough.”
Hope’s Alan Weatherill concurs with Paul Lew’s conclusion that ceramic bearings are not suited to the demands of cycling and says their suitability to industrial machinery doesn’t necessarily provide the performance benefit for cyclists that many people and companies claim they do.
“Another issue with using them [ceramic bearings] on bicycles is their hardness,” says Weatherill. “While this again is an advantage in many industrial applications, it's a major drawback on bikes. The shocks from hitting potholes and other road blemishes impact the hard ceramic balls into the softer steel races commonly used. This dent in the race is then felt when the bearing is rotated, giving you rough bearings.”