从pictureBox中裁剪绘制的矩形并将裁剪后的图像分配给pictureBox时为什么要放大？

Question

form1 top

Rectangle rect;
bool painting = false;

构造

this.pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage;

pictureBox1 mousedown事件

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                rect = new Rectangle(e.X, e.Y, 0, 0);
                painting = true;
            }
        }

picturebox1 mousemove事件

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                rect = new Rectangle(rect.Left, rect.Top, Math.Min(e.X - rect.Left, pictureBox1.ClientRectangle.Width - rect.Left), Math.Min(e.Y - rect.Top, pictureBox1.ClientRectangle.Height - rect.Top));
            }
            this.pictureBox1.Invalidate();
        }

picturebox1绘制事件

private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            if (painting == true)
            {

                using (Pen pen = new Pen(Color.Red, 2))
                {
                    e.Graphics.DrawRectangle(pen, rect);
                }
            }
        }

picturebox1 mouseup

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            timer1.Enabled = false;
            Image tmp = pictureBox1.Image;
            pictureBox1.Image = SaveRectanglePart(pictureBox1.Image, rect);
            if (tmp != null)
                tmp.Dispose();
        }

方法SaveRectaglePart

public Bitmap SaveRectanglePart(Image image, RectangleF sourceRect)
        {
            var bmp = new Bitmap((int)sourceRect.Width, (int)sourceRect.Height);
            using (var graphics = Graphics.FromImage(bmp))
            {
                graphics.DrawImage(image, 0.0f, 0.0f, sourceRect, GraphicsUnit.Pixel);
            }

            return bmp;
        }

问题是当我在pictureBox1上绘制一个矩形然后当它在鼠标向上事件时我想要它做的是将picturebox1中的当前图像替换为矩形内的图像我drawed。

在绘制的矩形中拉伸图像并将其分配给pictureBox1，现在在pictureBox1中我将从绘制的矩形内部获得裁剪后的图像。

但我在pictureBox1中获得的内容是放大了图片。无法弄清楚原因。

这是我绘制矩形时的截图

drawn rectangle

这是我在mouseup事件中获得的截图。

zoomed in