Odd grid sizing behaviour

Question

I have been using tkinter's grid geometry manager and have configured all the rows and columns to weight = 1. The problem comes when I try to resize a Tk() window. When I resize it vertically only, the width of widgets and the proportion of column widths also change despite their equal weight. Furthermore, the width of the columns isn't in the desired proportions (i.e. all equal) before doing any resizing - to be precise the first column (with labels and buttons) is as wide as other two (holding a canvas with columnspan = 2 and two labels).

I think the problem is somehow connected to the PiSimulator._adjust_sizes function which dynamically changes the font or to the use of the canvas with a columnspan of 2.

Here's my code:

import tkinter as tk
from tkinter import font    

class PiSimulator(tk.Frame):

    def __init__(self, root, **kwargs):
        super(PiSimulator, self).__init__(root, **kwargs)
        self.root = root
        self.root.title = 'Crtac krugova'
        self.root.minsize(width = 600, height = 400)
        self.pack(fill = 'both', expand = 1)
        self.custom_font = font.Font(size = 20)
        self.bind('<Configure>', self._adjust_sizes)
        self.populate()
        return

    def populate(self):

        tk.Label(self, text = 'Radijus:', font = self.custom_font, relief = 'raised',\
                        fg = 'green').grid(sticky='nswe', row = 0, column = 0)      
        self.radius = tk.IntVar()
        tk.Spinbox(self, from_ = 1, to = 16, font = self.custom_font, textvariable = self.radius).grid(row = 1, column = 0) 

        tk.Label(self, text = 'Greška', font = self.custom_font, relief = 'raised', anchor='w',\
                        padx = 5, pady = 5, fg = 'green').grid(sticky='nswe', row = 0, column = 1)

        self.error = tk.DoubleVar(0.0)
        tk.Label(self, textvariable = self.error, font = self.custom_font, relief = 'raised',\
                        anchor='e', pady = 5, fg = 'green').grid(row = 0, column = 2, sticky='nswe')


        self.canvas = PiCanvas(self, highlightthickness = 0)
        self.canvas.grid(row = 1, rowspan = 5, column = 1, columnspan = 2, padx = 5, pady = 5, sticky='nswe')

        tk.Button(self, text = 'Nacrtaj', font = self.custom_font, command = lambda: self.canvas.draw(self.radius.get())).grid(\
                        column = 0, row = 2, sticky='nswe')
        tk.Button(self, text = 'Izbriši',  font = self.custom_font, command = self.canvas.clear ).grid(\
                        column = 0, row = 3, sticky='nswe')
        tk.Label(self, text = 'POVRŠINA', relief = 'groove',  font = self.custom_font).grid(column = 0, row = 4, sticky='nswe')
        self.area = tk.IntVar(0)
        tk.Label(self, textvariable = self.area, relief = 'groove',  font = self.custom_font).grid(column = 0, row = 5, sticky='nswe')


        for i in range(3):
            self.columnconfigure(i, weight = 1)
        for i in range(6):
            self.rowconfigure(i, weight = 1)
        return

    def _adjust_sizes(self, event):
        new_font = int(event.height/15) - 2
        self.custom_font.configure(size = new_font)
        return


class PiCanvas(tk.Canvas):

    def __init__(self, root, **kwargs):
        super(PiCanvas, self).__init__(root, background = 'white', relief = 'groove',  **kwargs)
        self.root = root
        self.cubes = []
        self.circle = None

    def draw(self, radius):
        self.clear()
        c_width = self.winfo_width()
        c_height = self.winfo_height()
        self.grid_side = int(min([c_width, c_height]))
        self.grid_side-= 10
        self.v_offset = (c_height - self.grid_side)/2
        self.h_offset = (c_width - self.grid_side)/2
        self.cube_side = self.grid_side/(2*radius)
        vertix_coord = lambda x, x_cor, y_cor: x_cor*self.h_offset+y_cor*self.v_offset+x*self.cube_side

        for i in range(2*radius):
            new_line = []
            for j in range(2*radius):
                cube = self.create_rectangle(vertix_coord(i, 1, 0), vertix_coord(j, 0, 1),\
                                                vertix_coord(i+1, 1, 0), vertix_coord(j+1, 0, 1), tag = 'unused')
                new_line.append(cube)
            self.cubes.append(new_line)

        self.circle = self.create_oval(vertix_coord(0,1,0), vertix_coord(0,0,1),\
                                    vertix_coord(2*radius,1,0), vertix_coord(2*radius,0,1),width = 2)
        self.color_border_cubes()
        print(self.circle)

    def color_border_cubes(self):
        circle_coords = self.coords(self.circle)
        print(circle_coords)
        for i in self.cubes:
            for j in i:
                cube_coords = self.coords(j)
                if self.circle in self.find_overlapping(*cube_coords) + self.find_enclosed(*circle_coords):
                    self.itemconfigure(j, fill = 'green')


    def clear(self):
        self.cubes = []
        self.circle = None
        self.delete('all')      
        return


root = tk.Tk()
a=PiSimulator(root)
root.mainloop()