Hold on, hold on... you built a new data.tre that includes all the data as well as a data2 folder containing all the original files, and you're surprised that this new data.tre is twice the size of the original?
Another question - why are you trying to add these to data.tre in the first place? With a vanilla SO install, it's enough to place the files in the appropriate folder, and the game will automatically use them instead of the TRE file (so, if you've tried this, and it doesn't work, then you're doing something wrong - either the files are in the wrong place or have the wrong names). And if you're using UE, then you can either alter ue_tre.cfg to allow the game to use folder files (in which case, see above), or build a TRE file containing only the files you changed, and add this file at the very bottom of ue_tre.cfg (before 'end', of course), so that the game reads it last.