Pokemon-go – How to disengage from a Pokemon I don’t feel is worth capturing

Take a look at this article Pokemon GO: 9 Essential Tips, Tricks & Answers To Your Questions

How to capture Pokemon

1. Use the Item bag to toss Pokeballs or use items (like Razz berries) to make capturing a Pokemon easier. Improved Pokeballs such as Great Balls, Ultra Balls or Master Balls will also improve your chances.

2. The targeting circle tells you how tough the capture will be. A green circle is best, following by a yellow, orange, and red circle. Yellow circles, in our experience, are a pretty tough capture — orange and red? Wait until you’ve got the right items to make capturing easier.

3. Wait until the targeting circle is at its smallest before flinging a Pokeball. The smaller the circle, the better your chances of capturing will be.

Yes different types of pokemon are a different "distance" away from your pokeball. Flying pokemon such as zubats require a longer throw while small pokemon such as weedles and caterpies tend to require shorter distances. Other larger ground pokemon tend to be in between these two distances. CP also has some effect on distance where a higher CP weedle is further away than a lower CP weedle.

Also for whatever reason Pidgies and Spearows don't really fly so they can also be really close to the pokeball.

Source: Personal experience