In 1995, Alma College mathematician John F. Putz counted the measures in Mozart’s piano sonatas, comparing the length of the exposition (a) to that of the development and recapitulation (b):
Köchel and movement

a

b

a + b

279, I

38

62

100

279, II

28

46

74

279, III

56

102

158

280, I

56

88

144

280, II

56

88

144

280, II

24

36

60

280, III

77

113

190

281, I

40

69

109

281, II

46

60

106

282, I

15

18

33

282, III

39

63

102

283, I

53

67

120

283, II

14

23

37

283, III

102

171

273

284, I

51

76

127

309, I

58

97

155

311, I

39

73

112

310, I

49

84

133

330, I

58

92

150

330, III

68

103

171

332, I

93

136

229

332, III

90

155

245

333, I

63

102

165

333, II

31

50

81

457, I

74

93

167

533, I

102

137

239

533, II

46

76

122

545, I

28

45

73

547, I

78

118

196

570, I

79

130

209

He found that the ratio of b to a + b tends to match the golden ratio. For example, the first movement of the first sonata is 100 measures long, and of this the development and recapitulation make up 62. “This is a perfect division according to the golden section in the following sense: A 100measure movement could not be divided any closer (in natural numbers) to the golden section than 38 and 62.”
Ideally there are two ratios that we could hope would hew to the golden section: The first relates the number of measures in the development and recapitulation section to the total number of measures in each movement, and the second relates the length of the exposition to that of the recapitulation and development. The first of these gives a correlation coefficient of 0.99, the second of only 0.938.
So it’s not as impressive as it might be, but it’s still striking. “Perhaps the golden section does, indeed, represent the most pleasing proportion, and perhaps Mozart, through his consummate sense of form, gravitated to it as the perfect balance between extremes,” Putz writes. “It is a romantic thought.”
(John F. Putz, “The Golden Section and the Piano Sonatas of Mozart,” Mathematics Magazine 68:4 [October 1995], 275282.)