Can't help but think people arent actually answering OPs question (I could be wrong).
OP, does your salary come out of that £100k (soon to be £125k)?
Based on your question, you were basically asking for perspective on your salary VS the budget/ people you manage? And...if the fact you will now have to manage an additional £25k or 1 person should lead to you getting more money. To answer this question directly, the answer is no (in almost all instances).
As others have said, whilst the scope of what you're managing 'can be' slightly relevent to your salary, generally it doesnt have much correlation with how much you will be paid.
Back off topic somewhat - That really is a very low budget for 7 people!
Assuming this budget is split evenly (which it obviously isnt because its split between part time full time) -
£125,000 / 7 staff = £17,857
If you allow for Employers NI in this figure it looks something like this (Salary £16,660 + ENI £1,200)
Now if you make allowance for Pension contribution (if any), sick pay, holiday entitlement say this reduces the salary by £1,000.
This means 7 people are earning circa £15,600?
Whats alarming is if your salary isnt included in this - if you allowed for the part time staff earning approx 80% what the full time staff earn then this would mean that part time staff get £14,000 and full time staff get £17,000 approx. (although this allows for an even split of 3.5 part time 3.5 full time rather that 4 part time, 3 full time as I cant be bothered to work it out anymore lol). If your salary is included in this then these figures become much worse.