Are they the same thing?
Is downside deviation no more than the standard deviation?
They should be the same. However, I can’t imagine why, an investor might have a target semivariance designed to limit the variability of returns above a target value. To answer your second question, downside deviation could be more than the standard deviation for assets that have relatively stable returns over the mean but highly volatile when it underperforms. Also, the sample size will have an effect. If you have lets say a 10 returns below the mean but 1000 above then your downside deviation could be higher
downside deviation only calculates stuff below a certain target deviation number, and only for those parts of the returns that are below that number. so it really does nothing with numbers that are above the target. it may be a 1000, may be 10000 - it does not matter. if 10 are below in both cases - the downside deviation would be for those 10 numbers ONLY.