菜鸟请问ＶＢ绘图失真问题

围绕一个小圆画一圈大圆，为什么当“大圆半径”太大时，画出来的圆形失真？
Private Sub Command1_Click()

x = CDbl(x.Text)
y = CDbl(y.Text)
dybj = CDbl(dy.Text)
xybj = CDbl(xy.Text)

For i = 0 To CDbl(xsjd.Text) Step CDbl(fbl.Text)
    Pic1.Circle (x + (xybj + dybj) * Cos(i * 0.0174532925199), y + (xybj + dybj) * Sin(i * 0.0174532925199)), dybj

Next i

End Sub

Private Sub Command3_Click()
Pic1.Cls
End Sub

Private Sub Command4_Click()
x = CInt(x.Text)
y = CInt(y.Text)
dybj = CDbl(dy.Text)
xybj = CDbl(xy.Text)
Pic1.Circle (x, y), xybj, QBColor(15)

End Sub