Memory usage by Matlab - matlab

Question

I'm running a Matlab code in the HPC of my university. I have two versions of the code. The second version, despite generating a smaller array, seems to require more memory. I would like your help to understand if this is in fact the case and why.
Let me start from some preliminary lines:
clear
rng default
%Some useful components
n=7^4;
vectors{1}=[1,20,20,20,-1,Inf,-Inf];
vectors{2}=[-19,19,19,19,-20,Inf,-Inf];
vectors{3}=[-19,0,0,0,-20,Inf,-Inf];
vectors{4}=[-19,0,0,0,-20,Inf,-Inf];
T_temp = cell(1,4);
[T_temp{:}] = ndgrid(vectors{:});
T_temp = cat(4+1, T_temp{:});
T = reshape(T_temp,[],4); %all the possible 4-tuples from vectors{1}, ..., vectors{4}
This is the first version 1 of the code: I construct the matrix D1 listing all possible pairs of unordered rows from T
indices_pairs=pairIndices(n);
D1=[T(indices_pairs(:,1),:) T(indices_pairs(:,2),:)];
This is the second version of the code: I construct the matrix D2 listing a random draw of m=10^6 unordered pairs of rows from T
m=10^6;
p=n*(n-1)/2;
random_indices_pairs = randperm(p, m).';
[C1, C2] = myind2ind (random_indices_pairs, n);
indices_pairs=[C1 C2];
D2=[T(indices_pairs(:,1),:) T(indices_pairs(:,2),:)];
My question: when generating D2 the HPC goes out of memory. When generating D1 the HPC works fine, despite D1 being a larger array than D2. Why is that the case?
These are complementary functions used above:
function indices = pairIndices(n)
[y, x] = find(tril(logical(ones(n)), -1)); %#ok<LOGL>
indices = [x, y];
end
function [R , C] = myind2ind(ii, N)
jj = N * (N - 1) / 2 + 1 - ii;
r = (1 + sqrt(8 * jj)) / 2;
R = N -floor(r);
idx_first = (floor(r + 1) .* floor(r)) / 2;
C = idx_first-jj + R + 1;
end