Have you ever questioned how small Achievable goals are better than big goals? One feature that small goals provide better is high level of confidence. Having confidence while pursuing your goals is a major factor for successfully achieving them. As only confidence can let you work on your goals and can give you faith that you have the capability, skill etc. required for achieving it. And this confidence in turn helps in setting more and more powerful goals for achieving success in life. 
But now the question arises Why small goals are more powerful for increasing confidence than the big goals?
There are following points that proves how setting small Achievable goals boosts confidence:-
Achievable in short time:- Small Achievable goals are achievable usually within a few hours or days as compared to big goals. Due to this shortage of time, It gives a person a confidence that the goal set requires only a short amount of time, so instead of waiting for years he/she can put efforts now to achieve it. Also, This short time a sense of urgency in the mind of a person who wants to achieve his/her small goal and leads to more focus on the goal set. For Example:- If a student is given a test from a single topic of a chapter and the test is to be taken after 2 days then this will create an urgency in the mind of that student to complete it as other-wise timeline will pass soon and at the same time completing one topic will seem simple as compared to completing a whole chapter or book. Thus achievable in short time.
Gives Immediate Reward:- As Small goals often require a few hours or days for achievement, These also provides an immediate reward of the efforts put towards achieving them. This immediate achievement gives a sense of satisfaction that you received the reward of your efforts or hard work which in turn gives you confidence to further set more small goals or the big ones.
Leads to bigger goals:- A Big or ultimate goal which will take years to achieve, can be divided into a number of smaller goals. And achieving these small ones will lead you to the ultimate goal and a sense of progress that you are on the way to achieve the Ultimate goal set. For Example: You want to reduce 20 Kgs of fat which might take an year or so. You can set a goal to reduce 1 or 2 kg per month or even after a Week.  Decreases Uncertainity:- When a goal is Achievable within a short time, uncertainties that lies in long run vanishes. You can't definitely forecast what will happen after a year or two. But you have that much knowledge that what you will be doing after a few hours or tomorrow so you can work on the goals set at right time, which in turn strongers your belief that the goal set is achieveable due to lack of uncertainty. 
Stress/Anxiety Reduction:- Thinking about a big goals often leads to stress and anxiety as you cannot clearly see what can possibly happen in the long run. Also many doubts in your mind also come up like whether you can achieve that goal or not, whether you will get he reward you expecting, what difficulties can occur in the upcoming time etc. As these bigger goals require many things to work together for their achievement.
But on the other hand, smaller goals due to having less uncertainties requiring less efforts which in turn reduces unnecessary pressure on your mind and at the same time increases the confidence that the goals set are achievable.
Conclusion:- So the benefits like decreasing uncertainty, reducing stress/anxiety related to achieve-ability, achieve-ability in short time etc.  provided by small goals makes them unique and preferable over the big goals which take a lot of effort and time. But also It must be kept in mind that small goals are not enough as well to fulfill and satisfy anyone in the long run. So for this Big goals must be divided further into smaller chunks which require less efforts and time and by achieving them consistently, one can be on the path to success.