More Pins! or My Best Friend, the Seam Ripper

We are planning a wedding.  We have a budget.  A quite small budget in the world of wedding planning.

In fact, the budget is so small (in the world of wedding planning remember), the first wedding dress tried on and loved almost exceeded the budget all by itself.

Needless to say, that was not the actual purchased dress.  However, the actual dress, although almost second choice,  is much loved by the bride and is, in fact, becoming in danger of becoming a bit worn since it is frequently tried on by said bride.  Who cannot fasten her own dress and must call upon her mother.  'sigh.'  The mother is thinking seriously about finding a secret location for the dress and only telling one other person (not the bride) 'just in case.'

So, all that to say:  In order to save money and stay within the budget, the long-suffering bride's mother is making the bridesmaid's dresses.  Now the mother of the bride (whom shall be referred to from here on as MOB (I considered abbreviating 'bride's mother' but didn't want to be called BM)) volunteered to make the dresses, it was her suggestion.

She forgot the bride has no knowledge whatsoever of sewing.  The bride started out choosing bridesmaid's dressed that would require hospitalization of the MOB should she attempt the task.  Those patterns included lots of pieces and lots of notions--boning, special hem stuff, zippers, lining, magic and perseverance.

MOB took control and said, no, no, no.  Simple is better and more elegant. There will be no slippery icky fabric, there will be no more than ten pattern pieces.  There will be no crazy, order from somewhere else, notions.  (MOB is secretly hoping for magic to appear in the form of perhaps Cinderella's mice)

MOB has now made two practice dresses.  Can anyone please tell me why the pattern sizes do not coincide with our size in pre-made clothing?  And can someone tell me please why when I make the size of pattern the measurements suggest, the finished product is huge in some places but too small in others?  Both dresses are quite lovely, if I do say so myself and now the bride and MOH (maid of honor) have new dresses which they had better wear, not that I'm threatening or anything.