All of the notes have the same duration. You're overlooking the 3 by each group of three sixteenth notes. These are sixteenth-note triplets, of which there are twenty-four in a measure of 4/4 time. The broken second beam makes it clearer that there are groups of three, while the unbroken first beam fulfills the standard practice of beaming by beat.
Neither Wikipedia nor Dolmetsch online's music theory page has a good succinct explanation of triplet. The definition in Dolmetsch's dictionary is pretty good, but I don't think it's possible to link directly:
a group of three notes of equal time value performed in the time of two of them, however, (i) one or two of the notes may be rests of equivalent value, and (ii) a consecutive pair may be replaced by a note of double value.
Because three triplets take the time of two "normal" notes of the corresponding value, the duration of a triplet is 2/3 the duration of the corresponding note. For example, at quarter=50, a quarter note lasts 1200 milliseconds, a sixteenth note lasts 300 milliseconds, and a sixteenth note triplet lasts 200 milliseconds (in theory: if you are so precise in practice, the performance will be too mechanical sounding).