High Memory Usage with function files

I have created a code that do some calculations.

this code consists of a Script file and four functions.

Fun1, Fun 2 and Fun 3 output are one image for each. to create this image every function read around 100 images

these three images are used as input for Fun 4. Fun 4 read 500 image and do some calculations.

when i run this code, the memory usage in the first 3 functions is nearly 20% but when fun4 is being executed, the ram increases to 96%.

I tried to use clearvars at the end of each function but nothing happen.

i tried to use "pack" at the begining of the script file but nothing happen

Please note that i run this code before using script files instead of functions and the memory usage was much less.

I have to use the function files as there are more folders which require to run the code a lot and using function will let me to run it only one time.

So, what should i do to reduce this memory usage?