I was convinced of the importance of the lines below when Note8 was kneed in full HD mode.
Playing with a Mio C720 I went a little deeper into the problem.
No matter how light I put the cousin I got "Out of memory".
With the help of the lines below I managed to make a route Romania to Spain.
The calculation method is:
max_memory = Free RAM (obtained with ResInfo.exe)
reserve_memory = 1/8 max_memory
cache = 1/8 reserve_memory

Spoiler: some common values

[debug]

; RAM = 32 MB
;max_memory=33554432
;reserve_memory=4194304
;cache=524288

; RAM = 46 MB - Mio C720
;max_memory=48234496
;reserve_memory=6029312
;cache=753664

; RAM = 48 MB - Mio MOOV 500
;max_memory=50331648
;reserve_memory=6291456
;cache=786432

; RAM = 64 MB
;max_memory=67108864
;reserve_memory=8388608
;cache=1048576

; RAM = 99 MB - Mio S 605
max_memory=103809024
reserve_memory=12976128
cache=1622016

; RAM = 128 MB
;max_memory=134217728
;reserve_memory=16777216
;cache=2097152

; RAM = 256 MB
;max_memory=268435456
;reserve_memory=33554432
;cache=4194304

Attached a [Only registered and activated users can see links. ] for other max_memory values.
On line 3 enter the value of free RAM (obtained with ResInfo.exe).
On line 9 you get the three lines listed above (1 MB = 1048576).
On line 15 you get the three lines listed above (1 MB = 1000000).

Copy the contents of the corresponding cell to Notepad and from there into sys.txt.