:fyi: capital is unfortunately not the same as deposits.
A very simplified view with made up numbers might be: Monzo has £1 billion in customer deposits and £100 million of capital (Monzo’s own money from investors/ equity). When we lend out £1 million we need to hold £200k (20%) of capital (this is money we need to put aside and effectively do nothing with).
So you might think that Monzo can lend out £1.1 billion but this would mean we need to hold £1.1 billion * 20% = £220 million of capital which in this scenario we don’t have. 
So in this scenario we are capital constrained and whether we have £1 billion in customer deposits or £10 billion of customer deposits makes no difference. In fact, we actually need to hold capital on customer deposits as well so having savings pots and building customer deposits to £10 billion would actually be a bad thing.
Hope that makes senes - it’s a complicated topic!!