Its not about being fat.
At the end of the day most boards are designed to operate at their peak for the X percentile.
When someone is above 100kg they are outside the bell curve for most designs. Period.
It then becomes a game of finding a board that can handle the extra weight standing on it without destroying its performance.
Been there and done that-a lot.
For flat to mild conditions nothing, absolutely nothing, competes with the Ace GT.
The vertical tail doesn't drag through the water and the relatively flat rocker line/parallel rails translate to descent speed through the water.
There is more form and parasitic drag because you are sinking the board deeper into the water but your corresponding extra strength will inevitably overcome this.
In mild downwind conditions (or long period, windy downwind swell) they absolutely hammer.
Super stable.
For shorter period swell or ocean conditions go for an SIC 17'4" unlimited.
They are surprisingly nimble and quick in the flat but the rocker limits ultimate speed in the flat.
Give them downwind conditions and wind above 25 kts and you'll understand what a downwind religious experience is.
If I could only have one board this would be it.
Also very stable.
If you are limited to a 14' board then the choices are limited if you desire speed.
Yes, there are many 28"+ boards from manufacturers such as Naish, Starboard, NSP, et al but they are generally too heavy, slow or both.
At 105kg you need at least 300L and preferably 320+.
NSP make a fast, stable 26" dugout that is recommended as it is fast downwind and is as stable as most 28" boards. Their 28" and 29" are too slow for big boys unless you are paddling in big offshore conditions.
The 2016 Fanatic Falcon is arguably the fastest DW board on the market (although its on the small side volumetrically for a 105kg rider unless the conditions are BIG).
There are also options such as a custom from DC or Deep both of whom make good boards.
At the end of the day its going to come down to understanding the conditions you will spend the majority of your time in and matching a high volume board to those needs. Remember its stability before ability, compromise to maximise budgetry enjoyment, and time on the water. And if you love paddling paddling in BIG downwind conditions you could ride a 30" wide plank and have fun.
Happy paddling.