Can zucchini be increased in zucchini bread recipes?
Zucchini bread is a quick bread. It is chemically leavened and bound together mostly with egg but some gluten.

Adding more zucchini will add liquid and fiber which will interfere a little more with gluten development. Both of these changes will result in a bread that is more moist and tender- to the extreme of not holding together enough.

You can increase the zucchini- how much is difficult to answer because the bread will be different and how much is ok will be a matter of taste. I have found that for the quick breads I make, such as banana and zucchini, I can up to double the titular ingredient. 

The resulting bread is always very moist and dense. Sometimes it is too tender to slice. The flavor is more pronounced and sometimes that's what I'm in the mood for.

Interestingly, bread made in this way has a much shorter shelf life because of the extra moisture.

I've tried adding another egg to hold it together but that quickly becomes too "eggy". Of course adding more flour means you are just making a bigger batch of your original recipe.