The statistics indicate that your util_memoize cache is 200000 bytes, not MB! I do not understand the last sentence. What exactly is "it"?
Yet another attempt to help you:
Run the following script in your ds/shell:
foreach i {1 2 3 4} {util_memoize [list string repeat abcde 3998$i]}
append _ "nearly full: " [ns_cache_size util_memoize] " "
foreach i {4 3 2 1} {util_memoize [list string repeat abcde $i]}
append _ "over full: " [ns_cache_size util_memoize]
The first line fills your cache to the size 199931 bytes. The third line crosses in several steps the 200k border. Does this work on your machine?