Sicuro no, ma potrebbe esser il garbage collection che non riesce a cancellare i file di sessione obsoleti....

Se fosse linux potrebbero essere dei permessi.... ma con win

c'era un vecchio bug ... potrebbe trattarsi dell'impostazione di:

; Define the probability that the 'garbage collection' process is started
; on every session initialization.
; The probability is calculated by using gc_probability/gc_divisor,
; e.g. 1/100 means there is a 1% chance that the GC process starts
; on each request.

session.gc_probability = 1
session.gc_divisor = 1000

prova ad aumentare il session.gc_divisor e verifica anche come hai impostato il session.bug_compat

; PHP 4.2 and less have an undocumented feature/bug that allows you to
; to initialize a session variable in the global scope, albeit register_globals
; is disabled. PHP 4.3 and later will warn you, if this feature is used.
; You can disable the feature and the warning separately. At this time,
; the warning is only displayed, if bug_compat_42 is enabled.

session.bug_compat_42 = 0
session.bug_compat_warn = 1