My only thoughts would be too large of pipe would make it that much harder to "start" the siphon effect. Thanks for any more advise.
It is pretty easy to get a misconception of how the siphon comes into play.
During normal rainfall events it is unlikely it will go into "siphon" mode.
During normal rainfall events the pond will rise to the level the water will simply "run over" the overflow. This will happen with either a regular overflow pipe or a bottom draw style. With a bottom draw style the water column pressure will cause the overflow water to come from the bottom of the pond instead of the top. Both regular overflow pipes and bottom draw systems work the same way in this mode. The water simply flows over the highest point of the bottom of the inside of the pipe, which is the full pool level of the pond.
The overflow system can go into siphon mode whether it is a bottom draw system or an ordinary overflow pipe. For example, I have a simple 8" overflow pipe that is angled down to near the bottom of the backside of the dam. Pretty typical and conventional. It can go into siphon mode once the water reaches more than 8" above full pool and the top of the pipe is covered. The "siphon" volume of water that the pipe can flow is considerably more than regular flow rates because the "weight" of the water, because the pipe is angled down, will cause the water to flow faster. So once the pipe is full with all air out, assuming the pipe is angled downward, it will flow more water than what the same pipe will in a horizontal position (assuming a vortex does not introduce air at the top). This is siphon mode. The steeper the angle down and the longer the drop (higher head pressure or in this case vacuum), the greater the flow the pipe will handle in siphon mode. This is no different for a conventional overflow pipe or a "siphon" system.
Where the siphon system comes into play is the conventional overflow pipe is connected to a down pipe that extends to near the bottom of the pond on the water side. This is where you get water drawn from the bottom of the pond. But until the pond inflow gets high enough to cover the overflow pipe at the top (so no air is introduced), it does not go into siphon mode. It works just like a regular overflow pipe. The water simply rises and overflows, same as a regular overflow pipe, with the exception that the water comes from the bottom of the pond instead of the top. Neither system goes into siphon mode until water completely covers the highest point in the pipe (including the air vent in a siphon system). It only goes into siphon mode when no air can enter.
With a regular overflow pipe that is angled down, the siphon mode stops as soon as there is an air gap at the top of the pipe. Then regular overflow and overflow rates happen. With a bottom draw siphon system, there has to be a "vent" at the top of the pipe to break the siphon. Otherwise the siphon would continue until the pond was low enough where the inlet was near the same level as the outlet. This is why you have to make sure this "vent" does not get clogged - the siphon will drain your pond. In fact, this vent can be purposely shut off just for the provision of draining the pond.
Clear as mud? You may have already known this, but in case you did not, thought maybe this explanation would help. The experts can correct my technical mistakes. Siphon mode is not started, with either a siphon system or conventional overflow pipe angled down, until the entire pipe is covered with no air available to be introduced. In other words, in high rainfall events where the entire pipe gets covered. But the source of the water always comes from where the inlet is. In a bottom draw it comes from the bottom and in a conventional overflow it comes from the top. Hope this helps.