What you have to imagine is the fact, that waves when are (ideally) reflected from a wall lead to a standing wave. If in the wall is a tiny slit the components of the reflected waves left and right the slit dissipate in the direction of the slit (spherical waves in each point according to Huygens description). These components undermine the incoming wave at the slits position but not lead to a standing wave. So a little part of energy of the incoming wave goes inside the slit.
What happens there is the next dissipationdissipation prozess. We have to imagine that dissipation is always present in deep water (or you get some reflection from the ground). So the wave's energy in the tiny slit is fading very fast due to the small energy that could be transferred from the difference of the incoming and reflected waves. Then thinner the wall then less of the waves energy is fading.
As you pointed out the the size ratio between the wavelength and the slits wide is one of the reasons for does more or less of an incoming wave goes through this slit. The second reason is the thickness of the wall in the position of the slit.