Vocabulary

How does a linguist measure the vocabulary size? I tried to check my vocabulary seize on the web for various vocabulary check.. Depending on some tests, my Japanese vocabulary seize was about 65,000, and as for English, it was from 3,000 to 15,500. (English is a foreign language for me.)
  It was strange that the vocabulary seize wasn't fixed at all in English.
 Possibly, I suppose that I remembered several difficult words, though I missed easy ones or spelling.
 I always miss spelling in English very much. So, my score was so low when there are spelling checks in the test.
 By the way, I've not known these following words till now.

BUFFOON
INFLAMMABLE
SCURRY
SPIRITED

These are 13-14 year old level. (SAT native) 
 Whereas,
I answered correctly
MATRIX
PERSONA
SYSTEMIC
MIRAGE
ADJUNCT
SUPINE
These are high school graduated level (This level is too difficult for me.)