Hi @sugata_hisato,
The connectivity matrix (TF field) is saved as: [Nr x Ntime x Nfreq].
Nr represents the connectivity between two files, thus Na x Nb. However, the relation between Na x Nb and Nr may not be direct as it's optimized to save storage space for some connectivity metrics.
https://neuroimage.usc.edu/brainstorm/Tutorials/Connectivity#On_the_hard_drive
E.g. in correlation, as corr(a,b) == corr(b,a) only one value is saved. In your case 271*271 = 73441 elements; then the upper triangle of the connectivity matrix (36585 elements) plus the diagonal (271 elements) are saved, giving 36856 elements.
You can retrieve the connectivity matrix in the shape: [Na x Nb x Ntime x Nfreq] by using:
R = bst_memory('GetConnectMatrix', conn);
Best,
Raymundo