how enlarge display hdmi ipad to monitor

There is no way to change this.

iPads only output a certain size screen image

Some apps DO allow output a larger outputted image, but the iPad interface, itself will only output at a certain size based on the size of the screen being used to show the outputted content iPad content.

App screen output size varies by app and what app developers have decided for the outputted resolution of their apps.

It is what it is.

And Apple doesn't control what other third party app developers implement for screen size/resolution, either.

Like I stated, some apps will output at a larger screen resolutions, other apps may or may not.

What an app outputs for screen size/resolution can be app independent.

So some of your “beef” may lay with third party app developers and NOT, necessarily, Apple.