Operating with persitent vars can lead to the following compile info : "Not enough persistent memory"...
Most pleople facing the memory-comment won't agree intuitively since there is much memory left and the persitent-allocation is much less then the limits.
But the compiler-comment is related to the increase of additionally needed persitent memory. The compiler allocates for a persistent-variable a memory-range with additonally reserve of around 10%. That provides little flexiblity for smaller changes. In case the modificaton on the persitent var result in a bigger memory-space than available by the reserver the compiler-error pops up.
Since the memory needs to reorganiszed the system needs to get rid of the former allocation!