It's only a "£75 billion increase" over 6 years if you assume that spending would otherwise have been frozen in cash terms for 6 years - i.e. only if the government was, until today, planning to breach its NATO commitments. This is such an unhelpful way to present the figures.
To get the £75 billion number, the government has assumed a baseline with spending frozen in cash terms and then added up all of the differences. If you instead assume a baseline of spending frozen as a % GDP, it's an extra £20 billion over 6 years. Details here.
To make matters worse, when briefing the press the government said that this would "only" cost £4.4 billion in 2028/29. That assumes a baseline of 2.3% of GDP and so is inconsistent with the £75 billion number. They're just picking whichever baseline suits best.
A plea to whoever writes these press releases and social media posts:
1) stop using dodgy baselines
2) stop adding up the "extra" spending over an arbitrary period
3) if you can't do 1) and 2), at least be internally consistent..