It is due to how the hydrogen ion dissociates, strong mineral acids have complete dissociation whereas weak acids like citric acid don't. Don't be fooled into thinking that acids like formic are weak acids though, strong solutions of it and others can do a lot of harm to the unwary.
Gawd I may embarrass myself here but this doesn't make sense:
The strong acid should react more quickly as it fully dissociates. The weak acid releases H+ slowly into solution and the reaction with magnesium should be slower. However, as both acids are 0.1M, they both contain the same amount of H+ and over time the same amount of hydrogen gas is produced.