Out of Memory ///// Out of CPU ??? ?

Hello, 

Recently one of my applications has been getting bigger and bigger, and now approaches the point where I have about 128 kB  of memory use when it is running.

I have noticed now when going through the simulator that several watches fail due to not enough memory which is fine, but I have also noticed 

Those with 1275 Kb seem to run the app fine

Those with less than 1275... like the Forerunner 745 / Venu 3 / Venu 4 / with supposedly 1019Kb, do not appear to complete on compute functions for some reason.

They don't fail, they just don't seem to update some of the navigation dependent figures, I have no idea what's going on ? Is it something to do with the CPU chip ?

Maybe this list will help:

Forerunner 245  / 265 / 645 / 745 / 935 does not have enough memory / CPU to support the app any more. 
Vivoactive watches ( 3 and 4 ) , they boot but compute info so missing information ( track data ) 
Venu / Venu SQ not enough  memory
First Avenger - not enough memory
Fenix Chronos - Not enough memory  
Fenix 5 / quatix - not enough memory
Fenix 5S - not enough memory
Approach S62 - missing data calculation
Captain Marvel  - missing data calculation
Darth Vader  - missing data calculation
What Works in the watch simulator( I just tested  the 1275kB watches)  : 
FENIX 5 Plus  good
Fenix 5X good
Fenix 6 plus or PRO / tactix / delta X good
Forerunner 945 - Good
MARQ watches - Good
D2 Charlie / Delta / Delta PX / Delta S - Good  
Descent Mk1 - Good
Square displays:  ( not optimised )
GPS Mapp 86 - good ( not optimised )
GPS Mapp 66  - good  ( not optimised )
Montana 7 series -  good   ( not optimised )
Oregon 7 series -   good   ( not optimised )
Rino 7 series - Good  ( not optimised )
Edge Explore - Good  ( not optimised )
Edge 520 plus - Good  ( not optimised )
Edge 530 - Good  ( not optimised )
Any ideas?