Of course, this doesn't mean that constitutions are actually restraining in reality, for administration can always thwart the designs of a piece of paper; a court can aggrandize power for itself to "interpret" the constitution; or normal political processes can entirely subvert the supposedly eternal truths of a constitution in favor of transient political expediency. Moreover, a constitution itself can be nothing more than a political ploy, a way to show the world that democracy has been achieved while acting counter to its tenets. Of course, constitutions also need not be politically or economically liberal in any way shape or form: given that a constitution is written by political elites ascendant at the time, it can enshrine such anti-liberal attributes as slavery or restricted voting to a certain class/race/gender.
Moreover, the economic effect of constitutions is debatable; my own preliminary research in my book showed that constitutions really didn't matter in transition economies for many economic metrics, but this is somewhat to be expected (after all, constitutions shouldn't really matter in the short-term, and 20 years was definitely the short-term). We should really only expect to see constitutions have effects in the medium- and especially the long-run, as institutions develop under its protective umbrella. Of course, others have done much more intensive research in this area (Persson and Tabeliini literally wrote the book on this), but this has been somewhat limited to the structure of the political system that is created under a constitution; in the citation above, P&T find that presidential systems have smaller governments, while parliamentary systems have more persistent fiscal outcomes. Much more work needs to be done, especially in seeing if constitutions actually do restrain government in more than just size - based on the US, and given its explosion in size of government since the mid-1960s (but especially over the past 10 years), it is likely that current political processes can always undermine past ones.