Not saying it wasn't more tiring to you, but it's hard to believe the reason could be inertia. The difference between a light bike and a heavy bike is that the light one retains less inertia and everyone knows that a light bike is less tiring, not more.

Moreover, if you consider the physics of the situation it turns out that a smaller wheel does not actually have less inertia unless it's lighter because the faster spinning exactly cancels the smaller diameter. So it turns out that the weight and inertia of bicycle components are equivalent and the only way for inertia to make a bicycle less tiring is for all heavy bikes to be less tiring, which is ridiculous on its face.

This is another phrase that's thrown around a lot, but I find the meaning a bit cryptic. In engineering terms there's certainly nothing less stable about small wheels, so it's difficult to say whether any significance can be attributed to your feeling apart from being a matter of taste.

Unless, of course, they reasoned like you have here that "it must be best simply because other people are using it". And if a burgeoning bicycle designer like yourself thinks it reasonable to draw such a conclusion then why would you expect greater discernment from an ordinary rider?

And, as a designer, this is the part where you realize that even if smaller wheels are better they'd have to be substantially better to make you more money than giving in to the popular prejudice, which reveals the true advantage of larger wheels: marketability and economic efficiency.

The truth is that, as toys and sports equipment, most bicycles have not been designed with regard for what's best since the invention's infancy (ie before the invention of the automobile when bicycles were still considered vehicles). Contrary to popular option bicycle design is not highly evolved, but in a state of stunted growth because although many bicycle variations have been created, most were never sufficiently evaluated before the whole of human powered transport was relegated to toy status (ie for play or sport).

To assume that whatever is popular must be best is a slippery slope. Or as my father used to say, "the masses are asses".

Check http://www.mas-design.com/ for examples of where the same train of thought has taken the design of others.