Why many2one field in one2many is a littlte spacing odoo13

Question

I have 13 column in my one2many field and I can't reduce cause of requirement. The problem is my many2one column is very less space. When I remove some column , many2one field take perfect column width.But I can't reduce. How can I do that? I try to write css class for my tree view but not working.In image, I can only see Ac text but the really name is Account.

<page string="Yearly Budget">
    <field name="budget_yearly_line">
        <tree string="Yearly" editable="buttom" class="custom_class">
            <field name="account_id"/>
            <field name="jan"/>
            <field name="feb"/>
            <field name="march"/>
            <field name="april"/>
            <field name="may"/>
            <field name="june"/>
            <field name="july"/>
            <field name="auguest"/>
            <field name="sep"/>
            <field name="october"/>
            <field name="november"/>
            <field name="dec"/>
            <field name="amount"/>
        </tree>
    </field>
    <group class="oe_subtotal_footer oe_right">
            <field name="amount_untaxed"/>
        </group>
</page>