This is interesting.  According to Matlab documentation, if manual axis limits, data aspect ratio, and plotbox aspect ratio are all specified then  Matlab chooses to honor the first two (limits and data aspect ratio).  When all three are set in Octave it prioritizes data aspect ration and plot box ratio and sacrifices axis limits.

I checked axis.m in version 7.3 and it uses m-file code to find the minimum and maximum of the plotted data and then sets those limits.  On the development branch, this functionality has apparently been moved to C++ code.  The m-file code in axis.m is now

      elseif (strcmpi (opt, "tight"))
        ## sets the axis limits to the min and max of all data.
        set (ca, "xlimmode", "auto", "ylimmode", "auto", ...
                 "zlimmode", "auto", ...
                 "xlimitmethod", "tight", "ylimitmethod", "tight",
                 "zlimitmethod", "tight");

This is a regression: I tested with Octave 6.4 and it works as expected.

Code to reproduce is shown below and attached as tst_axis_tight.m.

[X,Y] = meshgrid(-3:6/17:3);
XX = 2*X.*Y;
YY = X.^2 - Y.^2;
colorscale = [1:18; 18:-1:1];
C = repmat(colorscale,9,1);

pcolor(XX,YY,C);

axis equal
axis tight