I have seen many posts describing the above fact, mostly regarding running iOS7 on older devices like the iPhone 4. One truth I found in all the stuff I read is the fact about how iOS7 loads its frameworks. As you all know iOS7 is designed for 64bit. Running 32bit apps might consume more memory than otherwise. Bit hard to believe, right? But it is true. This is how it happens.
iOS7 loads its frameworks to the memory and allows its apps to use it. This way each individual app is not required to load the required frameworks. By default iOS7 loads the 64bit frameworks. 32bit frameworks are loaded upon demand only. When loading a 32bit version is required additional memory get used up as now the memory has to hold both 32bit and 64bit versions.