Well inside low kicks to the lead leg are always done this way (unless a south paw vs orthodox). I presumed the OP meant leag leg round house kicks to the head or body.
To over simplify it. The round house kick from the back leg in Muai Thai/MMA is 'usually' thrown with very little bend on the knee - as the over used cliche goes 'its like swinging a baseball bat'.
The lead leg round house is more traditionally used in Traditional martial arts* - the knee is usually chambered and its more a 'snapping' movement compared to the 'swinging through' movement of the Thai Kick. so there is no way you cna generate the same power.
That being said I wouldn't like to be on the recieving end of GSPs or MAchidas front leg kicks :-)
*I'm not sure of this but I think Savate try to kick with a fairly straight lead leg.
You sir would be around 70% correct. Savateurs (practitioners of Savate) kick in a more traditional style where the legs are chambered and snapped into their opponents. This is due mostly to the fact that they kick with footwear on, and do not compete barefoot. As a result, the amount of damage that can be done by a pin-point strike from the hardened toe of a savateurs boot can be quite devastating even if it doesn't generate the same gross amount of power in comparison to a full MT Round Kick.
Also, it is possible to generate a great amount of power with a lead kick. Just like a lead hook in comparison to a rear straight/cross, the power is going to be generated primarily in your ability to ballistically torque your body over. The key is being able to pivot quickly and smoothly on your foot. I go into detail on that in this older post here
. The caveat is that you have to be able to do it explosively as you have less distance for the foot to travel and accelerate. Being able to do so takes a lot of practice.
The other way to create a lot of power off of your lead leg is to perform a quick shuffle or shift-step to momentarily change leads. In essence, the front leg is then "loaded up" and given the similar distance to travel as your rear leg while simultaneously moving your body mass into the direction of your opponent. This is exactly what GSP did to Matt Hughes in their second match.
While the second option easier to learn, you have to be able to shift step quickly, or be really good at setting your opponent up (GSP happens to be good at both). If a competent striker watches you load that kick they will either counter your attack with one of their own or get the heck out of dodge. While a great kick in itself, it would not have been anywhere near as effective if GSP had not spent the majority of the time before that softening up Hughes legs with low lead kicks.