It seems that esp32 only provide 4MB flash as read only rom.As you change the app0 partition size to 6.5MB,the esp linker under arduino can only produce the max size 4MB of your code.In the file \esp32\tools\sdk\ld\esp32_out.ld,the following lines declare the max size of the readonly flash rom.
/* Flash mapped constant data */
drom0_0_seg (R) : org = 0x3F400018, len = 0x400000-0x18