There's no advantage to being large when it comes to development other than the fact that cost for developing a province increases each time you do it.
This is entirely according to design.
On the contrary, the "advantage" (to a province) is if the owner is small.
Take 3 nations. For arguments sake, they all have exactly the same MPs available to them, and are all the same tech group.
Nation 1 is a 4PM, always will be, and nobody bothers them.
Nation 2 is a 4PM too, but is conquering, and is averaging about a new province per decade give or take.
Nation 3 is a large major power with 12 starting provinces who takes multiple new territories every few years and is feeding vassals which are then integrated.
How will development pan out?
Well, Nation 1 has tons and tons of excess monarch points, and only a whopping 4 provinces to funnel them into. They aren't coring, they aren't annexing, and they aren't spreading the love around much - those 4 regions should expect to become incredibly wealthy, with tons and tons of points inefficiently boosting their development.
Nation 2 is going to develop each province less than nation 1. It has to spend monarch points to core new land, and it is slowly increasing the number of lands with development to be put into them. It is probably still developing, but not as much as nation 1 per individual province, since it will want to put some into new land which costs less, since costs increase each time you develop. Due to efficiency rules it may end up with more total development, but individual provinces themselves will not be as developed as an individual province of the 4PM Nation 1.
Nation 3 won't be developing much if at all. Maybe just manpower, and even then, it's liable to spread it all over the place. It needs its dip points for annexing and its admin for coring; what's left for territories, and out of whatever is, why would it go repeatedly into the same place when it can be spread in smaller amounts across a large area?
To put it another way, if 2 nations each have 10k lifetime mp to use for development, the one with less provinces will develop those provinces more than the one with more provinces on a per province basis. Since monarch points are largely independent of nation size, the less provinces a nation has the more it will develop each individual one, with advisors merely helping level this slightly but not liable to fix the imbalance.