Learn English – the difference between “undergo” and “have been through”

• have had to
• had had to

These are both perfect constructions, which you may read about in stupefying (yet inadequate) detail here.

The perfect is constructed with a form of HAVE as an auxiliary verb followed by the past participle of the lexical verb. In these cases, the lexical verb is also HAVE, employed in the construction HAVE to ≈ must. The past participle of HAVE is had, so these constructions end with had to. In have had to ... the auxiliary is cast in the present-tense form, which is have with a first-person subject, so this is a present perfect. In had had to ... the auxiliary is cast in the past-tense form, which is always had, so this is a past perfect.

Examples of the use of these constructions:

• I have had to explain the perfect construction many times.

  When I say or write that sentence I mean that explaining the perfect construction many times is a component of my present history and experience.

• Two minutes ago I wrote that I had had to explain the construction many times.

  That sentence means that explaining the perfect construction many times was a component of my history and experience at that time in the past.

Generally, they mean the same thing - more than average of size, amount, shape, weight and the like.

Though they mean the same, it depends on the context that whether or not you can use those words interchangeably. In your case, they mean the same.

However, if I ignore your example, and focus on your main question 'what's the difference between largest and biggest', I'd say that use them with caution. They are not interchangeable in all cases.

'Big' may also mean 'important' but 'large' does not.

Buying a house was the biggest largest decision I took this year.

There are many other meanings of 'big'. Also, these words are not used in some cases.

Good read is here on the BBC English Learning page.