Pinning cloth can take a bit of practice to get used to.
In answer:
Yes they can be pinned tighter, maybe not quite as tight as a disposable could be (disps can go pretty tight if you try), they will also loosen a little as you move around. Also depending on what exactly you are using (flat, contoured, prefold etc.) and/or the type of fold you use, you can jelly roll them to make them more snug around the thighs and waist.
Pinning can be made easier with curved diaper pins like
These, they are safety pins that have a slight bend or curve to them designed for quilting to make it easier to angle back through the fabric. It's all just practice to get the feel for how to do it.
It can also be helpful to keep a bar of soap handy, carefully pushing the pin into the soap and taking it out will lubricate it and help it to slide more easily between the layers of fabric.
There is an excellent video on
This Site at the top on the page that demonstrates how to pin a diaper on a baby.
The pin does not need to go all the way through the fabric, you want to put your fingers inside the diaper between your skin and the fabric, pull the wing around then slowly push the pin at a slight angle toward your fingers, when you start to feel the pressure of the pin but not the point of the pin itself is when you angle the pin back upwards and push it back to the front. repeat on the other side pulling the wing as snug as you can, more than you think will be enough as it will loosen a little.
When I first used pins I thought they should be vertical (or close to) so that the diaper would not slide along the pin but this causes them to twist under the pressure, They should be inserted horizontally.
You shouldn't need more than one pin per side.
Make sure you use heavy duty or hardened pins as the extra weight and pressure on an adult diaper is much more than a babies and can bend cheaper/weaker pins.
Make sure they have a decent locking head as you don't want it popping out while wearing, there are some newer pins like
These that are called diaper pins but I would not want them anywhere near one.
Use Youtube, there are loads of cloth diapering videos out there showing different folds and techniques, most will use snappis as these are incredibly good but there are many with pins too.
Buy a smaller (childrens) cloth nappy in a similar style to what you use (flat, prefold, Contour) and find a teddy large enough for it to fit and practice pinning technique on that. It is a totally different angle than doing it on yourself but will help you get the feel for the pressure, how tight you can pull it and where to position them becauses you will actually be able to see it all much better.
I'm sure there are other tips from other members out here too but I hope that helps.