Ye, the seats, tables and legroom is all the same in first and standard class. The only difference space wise from first and standard on Scotrail 158s is that the airline seats have more legroom.
Indeed, it's something which solely affects Scotrail 158s, more specifically the Inverness based units. The reading lamp sits a couple of centimetres out from the window onto the table, thus compromising around 1/3 of the (very little) table space for window seats as opposed to standard where there is no lamp. It is impossible to open a newspaper, laptop etc at the table on any of the window seats because of this. So technically speaking by sitting in first class at a table, you are actually getting far less table space than you would in standard. The disappointment on a first class passenger's face when a 158 turns up on an Intercity service instead of a 170 is all too real.
See attached for comparisons, then you'll see