Love big boobs, lad? That's more likely than not (4-2)
I believe the answer is:
odds-on
'more likely than not' is the definition.
(I've seen this before)
'love big boobs lad?' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'big boobs' becomes 'dd' (I can't explain this - if you can you should believe this answer much more).
'lad?' becomes 'son'.
'o'+'dd'+'son'='ODDS-ON'
'that's' is the link.
Can you help me to learn more?
(Other definitions for odds-on that I've seen before include "With success more likely than favour - ask bookie" , "High probability" , "With success more likely than failure - check with bookie" , "Better than evens" , "With success more likely as bookie would calculate" .)