Can anyone help me

when i'm comparing two data sizes i'm having some trouble

i'm comparing 64 MB with 32 MB and my program correctly identifies that 64 MB is the larger size

but when i compare 64 MB with 512 MB it tells me that 64 MB is the larger size

heres my code can anyone amend it so that it compares the two data sizes correctly?

Code:
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If TextBox1.Text > TextBox2.Text Then
            MessageBox.Show("TextBox1 is the biggest")
        Else
            MessageBox.Show("TextBox2 is the biggest")

        End If
    End Sub