All studies to  investigate this have been done on two groups of patients in which a single surgeon employs the two approaches in the diff groups.

When a surgeon who is capable of doing a minimally invasive approach does a conventional approach it is logical that the conventional technique will be only marginally bigger and therefore advantages do not show up in studies. However if a minimally invasive approach of a surgeon is compared with a conventional approach of another surgeon who never does minimally invasive or never makes an attempt to reduce his incision size  (within comfort levels)- the differences will show up.

When one compares an incision which is 5 cms for a particular procedure with another which is 50 cms for the same procedure  - the differences will show up without any doubt.
However to see objective difference between an incision which is 5 cms and 8 cms it is difficult   This is a question of degree.

MIS approach has been accused to be just a marketing trick which has caused more harm than good. This is true in many instances however one must be careful not to confuse MIS surgery  with the concept of minimizing incision size
When surgeons are focused on  doing a surgery with a pre- determined incision size like say 10 cms - they are hell bent on doing this through this incision even though they are struggling and probably getting many things wrong in the deep bone work. This is certainly not good. Scientific papers enumerating surgical disasters when this is employed is common place
 
The other side of the coin is when surgeons chop up patients to extraordinary lengths. Certainly it is equally wrong to cut up tissues unnecessarily when the same  can be  accomplished  to the same degree of accuracy by employing a much smaller incision. In other words it is certainly the duty of the surgeon to minimize the length of incision of any elective procedure but ensuring that he is comfortable and deep bony work is not compromised in any way. There should not be any predetermined length but the surgeon must consciously reduce incision size as a guiding principle.
Undoubtedly a hip incision that goes all the way to the knee will have many other bad effects apart from the scar.
 
Therefore there is no doubt that surgeons must be constantly striving to reduce incision size without compromising any other factor. However trying to work with a pre-determined incision size is frequently a recipe for disaster. It is also well  accepted that  revolutionary techniques like the two incision technique for THR in which  the surgeons previous experience with THR is rendered completely useless is very risky when compared evolutionary techniques in which surgeons reduce incision size progressively.
 
Surgical speed is another interesting topic. The fastest hand that I have seen wield the scalpel is undoubtedly Ronan Treacy who can finish a resurfacing in 20-25 mts. However Mr. McMinn who invented resurfacing and who of course trained Mr. Treacy still takes close to two hours. The turnover time will be 3 hrs.
I still take close to two hrs for a resurfacing with a turnover time of 3 hrs. There are so many steps and no matter how fast you do them it takes that amount of time to do all the steps. The neck capsule preservation that i do takes extra time as well. Attempting to reduce incision size and using subcuticular absorbable stitches all add up the time taken for surgery. If I don’t do all these I probably can finish in an hour.

If I should finish a resurfacing within half an hour there is no doubt I will be skipping steps.