Rice is notoriously difficult to cook perfectly. If you cook rice on the stove-top, this video reveals that the secret to avoiding mushy rice might be in the way you figure out the water-to-rice ratios.
If a type of rice calls for a 1:1 ratio of water to rice -- one cup to one cup -- you'd think that doubling the recipe would just mean you use two cups of each. This video, from the America's Test Kitchen YouTube channel, suggests that rice-to-water ratios can't be scaled up proportionally, however, and that it may be why you're getting mushy rice every time you cook a big batch.
The ATK testing revealed evaporation doesn't double when you double the quantity of rice you're cooking -- it stays the same. So when you double a recipe, you have to account for that evaporation when cook it. Of course, you could always just spring for a nice rice cooker too.